core: Improve readability of _gpgrt_argparse.

* src/argparse.c (_gpgrt_argparse): Use enum for the states and
replace continue and break.

The use of continue and break in the endless loop if confusing due to
the size of the loop's body and because there are also inner for
loops with break. It is better to make things explicit by using goto.

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