po: Make agent/commands.c translatable and update German translation.

This also fixes a bad fuzzy translation which accidently had been
commited without realizing that there was indeed a change (from
"... GnuPG" to "... %s").

(cherry picked from commit 5ed1567e7c6b08988a134effb3c1f42ef6d5319a)

• GnuPG-bug-id: T4777