(GetPW): Add parameter to (dis)allow printing of Windows username.
Use defines instead of numerical constants where possible.
Try avoiding impersonation problem. Rearrange to print Windows
username only if it's different from Cygwin username.
(ChangePW): Use defines instead of numerical constants where possible.
(main): Call GetPW with additional parameter. Change error text.
* passwd.c (GetPW): Handle case of user-edited /etc/passwd
with cygwin_internal (CW_EXTRACT_DOMAIN_AND_USER, ...).
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