gpgconf: No ENOENT warning with --change-options et al.

gpgconf: No ENOENT warning with --change-options et al.

* tools/gpgconf-comp.c (retrieve_options_from_program): Check ERRNO
before printing a warning.
--

It is common that a conf files does not exist - thus we should not
print a warning.

GnuPG-bug-id: 2944

BTW: The error messages in gpgconf should be reworked to match those
of the other components.

Signed-off-by: Werner Koch <wk@gnupg.org>
(cherry picked from commit 30dac0486b6357e84fbe79c612eea940b654e4d1)

Gbp-Pq: Name 0036-gpgconf-No-ENOENT-warning-with-change-options-et-al.patch