dup2: work around mingw and cygwin 1.5 bug

dup2: work around mingw and cygwin 1.5 bug

* m4/dup2.m4 (gl_FUNC_DUP2): Detect mingw bug.
* m4/unistd_h.m4 (gl_UNISTD_H_DEFAULTS): Add witness.
* modules/unistd (Makefile.am): Substitute it.
* lib/unistd.in.h (dup2): Declare the replacement.
* lib/dup2.c (dup2) [REPLACE_DUP2]: Implement it.
* doc/posix-functions/dup2.texi (dup2): Document the bugs.
* lib/fchdir.c (rpl_dup2): Don't collide with mingw replacement.
* modules/execute (Depends-on): Add dup2.
* modules/fseterr (Depends-on): Likewise.
* modules/pipe (Depends-on): Likewise.
* modules/posix_spawn-internal (Depends-on): Likewise.

Signed-off-by: Eric Blake <ebb9@byu.net>