* learn:
doc: add public-inbox-learn(1) manpage
mda: support multiple List-ID matches
mda: prepare for multiple destinations
inboxwritable: add assert_usable_dir sub
mda: skip MIME parsing if spam
mda: hoist out mda_filter_adjust
filter/base: remove MAX_MID_SIZE constant
mda: hoist out List-ID handling and reuse in -learn
learn: hoist out remove_or_add subroutine
learn: GIT_COMMITTER_<NAME|EMAIL> may be "" or "0"
learn: update usage statement
learn: only map recipient list on "ham" or "rm"
learn: support multiple To/Cc headers