AC_TYPE_UID_T: Rewrite using AC_CHECK_TYPE.

AC_TYPE_UID_T: Rewrite using AC_CHECK_TYPE.

AC_TYPE_UID_T uses AC_EGREP_HEADER to search sys/types.h for
occurrences of the string ‘uid_t’ and, if found, assumes both
uid_t and gid_t are available.  This would be better done using
a pair of AC_CHECK_TYPE operations.

I also converted two uses of old-style AC_CHECK_TYPE, immediately
below, to new-style.  (There are probably other old-style uses in
this file, I only did the ones I happened to see.)

* lib/autoconf/types.m4 (AC_TYPE_UID_T): Check for uid_t and gid_t,
  separately, using AC_CHECK_TYPE, instead of grepping sys/types.h.
  (AC_TYPE_SIZE_T, AC_TYPE_SSIZE_T): Use new-style AC_CHECK_TYPE.