Cygwin: Add winsymlinks:sys

Add winsymlinks:sys, to explicitly select always using plain files with
the system attribute containing a magic cookie to represent a symlink.

winsup/cygwin/environ.cc

@@ -82,6 +82,8 @@ set_winsymlinks (const char *buf)
     allow_winsymlinks = WSYM_lnk;
   else if (ascii_strncasematch (buf, "lnk", 3))
     allow_winsymlinks = WSYM_lnk;
+  else if (ascii_strncasematch (buf, "sys", 3))
+    allow_winsymlinks = WSYM_sysfile;
   /* Make sure to try native symlinks only on systems supporting them. */
   else if (ascii_strncasematch (buf, "native", 6))
     allow_winsymlinks = ascii_strcasematch (buf + 6, "strict")

winsup/cygwin/globals.cc

@@ -57,7 +57,8 @@ enum winsym_t
   WSYM_lnk,
   WSYM_native,
   WSYM_nativestrict,
-  WSYM_nfs
+  WSYM_nfs,
+  WSYM_sysfile,
 };
 
 exit_states NO_COPY exit_state;

winsup/cygwin/path.cc

@@ -2071,6 +2071,7 @@ symlink_worker (const char *oldpath, path_conv &win32_newpath, bool isdevice)
 	  /* On FSes not supporting reparse points, or in case of an error
 	     creating the WSL symlink, fall back to creating the plain old
 	     SYSTEM file symlink. */
+	  wsym_type = WSYM_sysfile;
 	  break;
 	default:
 	  break;
@@ -2211,7 +2212,7 @@ symlink_worker (const char *oldpath, path_conv &win32_newpath, bool isdevice)
 		  * sizeof (WCHAR);
 	  cp += *plen;
 	}
-      else
+      else /* wsym_type == WSYM_sysfile */
 	{
 	  /* Default technique creating a symlink. */
 	  buf = tp.t_get ();

winsup/doc/cygwinenv.xml

@@ -76,11 +76,17 @@ in addition to the normal UNIX argv list.  Defaults to not set.</para>
 </listitem>
 
 <listitem>
-<para><envar>winsymlinks:{lnk,native,nativestrict}</envar> - if set to just
+<para><envar>winsymlinks:{lnk,native,nativestrict,sys}</envar></para>
+
+<itemizedlist mark="square">
+<listitem>
+<para>If set to just
 <literal>winsymlinks</literal> or <literal>winsymlinks:lnk</literal>,
 Cygwin creates symlinks as Windows shortcuts with a special header and
 the R/O attribute set.</para>
+</listitem>
 
+<listitem>
 <para>If set to <literal>winsymlinks:native</literal> or
 <literal>winsymlinks:nativestrict</literal>, Cygwin creates symlinks as
 native Windows symlinks on filesystems and OS versions supporting them.</para>
@@ -92,9 +98,21 @@ some reason, it will fall back to creating Cygwin default symlinks
 with <literal>winsymlinks:native</literal>, while with
 <literal>winsymlinks:nativestrict</literal> the <literal>symlink(2)</literal>
 system call will immediately fail.</para>
+</listitem>
+
+<listitem>
+<para>If set to <literal>winsymlinks:sys</literal>, Cygwin creates symlinks as
+plain files with the <literal>system</literal> attribute, containing a magic
+cookie followed by the path to which the link points.</para>
+</listitem>
+</itemizedlist>
+
+<para>Note that this setting has no effect where Cygwin knows that the
+filesystem only supports a creating symlinks in a specific way.</para>
 
 <para>For more information on symbolic links, see
 <xref linkend="pathnames-symlinks"></xref>.</para>
+
 </listitem>
 
 <listitem>

winsup/doc/pathnames.xml

@@ -389,17 +389,26 @@ ways.</para>
 <itemizedlist mark="bullet">
 
 <listitem>
-<para>The default symlinks created by Cygwin are either special reparse
+  <para>The default symlinks created by Cygwin are:</para>
-points shared with WSL on Windows 10, or plain files containing a magic
-cookie followed by the path to which the link points.  The reparse point
-is used on NTFS, the plain file on almost any other filesystem.</para>
 
-<note><para>Symlinks created by really old Cygwin releases (prior to
+  <itemizedlist mark="square">
-Cygwin 1.7.0) are usually readable.  However, you could run into problems
+    <listitem>
-if you're now using another character set than the one you used when
+      <para>special reparse points shared with WSL (on NTFS on Windows 10 1607
-creating these symlinks (see <xref linkend="setup-locale-problems"></xref>).
+      or later)</para>
+    </listitem>
+    <listitem>
+      <para>plain files with the <literal>system</literal> attribute, containing
+      a magic cookie followed by the path to which the link points.
+      </para>
+      <note><para>Symlinks of this type created by really old Cygwin releases
+      (prior to Cygwin 1.7.0) are usually readable.  However, you could run into
+      problems if you're now using another character set than the one you used
+      when creating these symlinks (see <xref
+      linkend="setup-locale-problems"></xref>).
       </para></note>
     </listitem>
+  </itemizedlist>
+</listitem>
 
 <listitem>
 <para>On filesystems mounted via Microsoft's NFS client, Cygwin always