gpg-interface: do not hardcode the key string len anymore

gpg-interface: do not hardcode the key string len anymore

gnupg does print the keyid followed by a space and the signer comes
next. The same pattern is also used in gpgsm, but there the key length
would be 40 instead of 16. Instead of hardcoding the expected length,
find the first space and calculate it.
Input that does not match the expected format will be ignored now,
before we jumped to found+17 which might have been behind the end of an
unexpected string.

Signed-off-by: Henning Schild <redacted>
Signed-off-by: Junio C Hamano <redacted>