Move the duplicated search_key function to pam_modutil.

Move the duplicated search_key function to pam_modutil.

* libpam/pam_modutil_searchkey.c: New source file with pam_modutil_search_key().
* libpam/Makefile.am: Add the pam_modutil_searchkey.c.
* libpam/include/security/pam_modutil.h: Add the pam_modutil_search_key() prototype.
* libpam/libpam.map: Add the pam_modutil_search_key() into a new version.
* modules/pam_faildelay/pam_faildelay.c: Drop search_key() and use
  pam_modutil_search_key().
* modules/pam_umask/pam_umask.c: Likewise.
* modules/pam_unix/support.c: Likewise.