Portability fixes.

XXX: libSystem.pc still contains WRKSRC references.
tell Cygwin and Haiku to pkglint.
Import user-20130306 as sysutils/user_cygwin.

This implements a subset of useradd(8)/groupadd(8) functionality on
Cygwin that is sufficient for pkgsrc use.
Added sysutils/user_cygwin version 20130306
+ user_cygwin
Switch to use own user_cygwin for _USER_DEPENDS.