Cygwin: path_conv: add PC_SYM_NOFOLLOW_DIR flag

Cygwin: path_conv: add PC_SYM_NOFOLLOW_DIR flag

Usually a trailing slash requires to follow an existing symlink,
even with PC_SYM_NOFOLLOW.  The reason is that "foo/" is equivalent
to "foo/." so the symlink is in fact not the last path component,
"." is.  This is default for almost all scenarios.

PC_SYM_NOFOLLOW_DIR now allows the caller to request not to
follow the symlink even if a trailing slash is given.  This can
be used in callers to perform certain functions Linux-compatible.

Signed-off-by: Corinna Vinschen <corinna@vinschen.de>