Back-port of 'keygen.keygrip' via '--command-fd' to 2.2

[Ulrich Buchgraber]

The 'keygen.keygrip' input has been missing for '--command-fd' and
this is already fixed in 2.3 with commit 19b1a28621.

There referenced GnuPG-bug-id is 5771.

Would it be okay to back-port this to the stable 2.2 branch?

I don’t see a breaking change because the keygen.algo=keygrip answer
wouldn’t work anyways without this fix (would just stop with a tty
input for the keygrip).

Greetings, Ulrich