util/crypto: _n_m_crypto_potential_payload: rename "payload" arg to "part"

_notmuch_message_crypto_potential_payload is called on a GMimeObject
while walking the MIME tree of a message to determine whether that
object is the payload.  It doesn't make sense to name the argument
"payload" if it might not be the payload, so we rename it to "part"
for clarity.

This is a non-functional change, just semantic cleanup.

Signed-off-by: Daniel Kahn Gillmor <dkg@fifthhorseman.net>