gpg-connect-agent: Catch signals so that SIGPIPE is ignored.

* dirmngr/server.c (cmd_killdirmngr): Return 0.
* tools/gpg-connect-agent.c (main): Catch signals.

And we also print nicer diagnostics. The reason we need this is that
for example "gpgconf --kill dirmngr" uses gpg-connect-agent to send a
command to dirmngr. This may results in a SIGPIPE which in turn leads
to an annoying error message from gpgconf.

• Signed-off-by: Werner Koch <wk@gnupg.org>