tools: Add comments explaining the functions parameters.

* tools/gpgconf-comp.c (change_options_file): Add comments explaining
the functions parameters.
(change_options_program): Likewise.

From reading those two functions it is not clear why we write the new
configuration to the source file, and read the current configuration
from the destination file. The identifiers are named for the benefit
of the callee.

Note that identifiers that require that much explanation are poorly
chosen.

• Signed-off-by: Justus Winter <justus@g10code.com>