common: Fix gnupg_reopen_std.

* common/sysutils.c (gnupg_reopen_std): Use fcntl instead of fstat.

When gpg was invoked by a Perl web application on FreeBSD, fstat in
gnupg_reopen_std failed with EBADF. Using fcntl, which is considered
lighter than fstat, it works fine. Since uur purpose is to check if
file descriptor is valid or not, lighter operation is better.

• Reported-by: Marcin Gryszkalis <mg@fork.pl>
• Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>