gpgconf: create local option file even if a global file exists.

* tools/gpgconf-comp.c (munge_config_filename): New.
(change_options_program): Call it.

In case a system-wide config exists but no local file, the component
returns the name of the system-wide config file and gpgconf tried to
update this file. This fixes this by detecting the presense of a
system-wide config file.

This applies only to 2.2; the configuration system in 2.3 has been
heavily changed to better handle system-wide configuration files.

• GnuPG-bug-id: T5650