Make key=value argument list optional for pkey_{encrypt,decrypt,sign}

keyctl's help message suggests that including a key=value style list
of arguments is optional for the pkey_* operations, and for pkey_query
and pkey_verify it indeed seems to be optional, but the other three
operations require that at least one key=value pair be passed in.

This patch changes the logic to make key=value lists optional for
all pkey_* operations.

Signed-off-by: Lennert Buytenhek <buytenh@wantstofly.org>
Signed-off-by: David Howells <dhowells@redhat.com>