common: New function decode_c_string.

* common/miscellaneous.c (decode_c_string): New.

This is basically a copy from the code we use in gpgme and gpa.

(cherry picked from commit 6ecedd0b25b6b1a33be63b99f2a8256370000521)

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