makedocbook: Fix false report of unhandled texinfo command

During 'make man', makedocbook falsely reports "texinfo command
'@modifier' remains in output" while processing the setlocal(3) manpage,
which contains that literal string.

Move the check for unrecognized texinfo commands to before processing
'@@' (an escaped '@') in the texinfo source, and teach it to ignore
them.

Improve that check slightly, so it catches non-alphabetic texinfo
commands, of which there are few.

Now we don't have false positives, we can make unrecognized texinfo
commands fatal to manpage generation, rather than leaving them verbatim
in the generated manpage.

newlib/doc/makedocbook.py

@@ -450,9 +450,6 @@ command_dispatch_dict = {
 def line_markup_convert(p):
     s = p
 
-    # process the texinfo escape for an @
-    s = s.replace('@@', '@')
-
     # escape characters not allowed in XML
     s = s.replace('&', '&')
     s = s.replace('<', '&lt;')
@@ -482,6 +479,14 @@ def line_markup_convert(p):
     # very hacky way of dealing with @* to force a newline
     s = s.replace('@*', '</para><para>')
 
+    # fail if there are unhandled texinfo commands
+    match = re.search(r'(?<!@)@[^@\s]+', s)
+    if match:
+        sys.exit("texinfo command '%s' remains in output" % match.group(0))
+
+    # process the texinfo escape for an @
+    s = s.replace('@@', '@')
+
     if (verbose > 3) and (s != p):
         print('%s-> line_markup_convert ->\n%s' % (p, s), file=sys.stderr)
 
@@ -823,10 +828,6 @@ def main(file):
 
     print(s)
 
-    # warn about texinfo commands which didn't get processed
-    match = re.search(r'@[a-z*]+', s)
-    if match:
-        print('texinfo command %s remains in output' % match.group(), file=sys.stderr)
 
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