agent: Make "PKDECRYPT --kem" with optional value work.
* agent/command.c (cmd_pkdecrypt): Fix comparison. * agent/agent.h (enum kemids): Rename type and strip trailing comma. * agent/pkdecrypt.c (agent_hybrid_pgp_kem_decrypt): Allow building with Libgcrypt < 1.11
Eventually we should change the libgcrypt requirement in configure.