build: Simplify for string.h and getopt.h.

* configure.ac (AC_CHECK_HEADERS): Remove string.h and getopt.h.
* dirmngr/ks-engine-ldap.c: Remove including getopt.h.
* tools/make-dns-cert.c: Likewise.

Checking string.h is supported by AC_HEADER_STDC. Use of getopt.h
is only needed for getopt_long of GNU extention.

• Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>