Enable rsync checksums when doing forced updates.

When forcing an update, also enable rsync checksums instead of relying
on the default comparison of only size and timestamp.  This allows
recovery of a repo with one or more incomplete or corrupt files with
correct metadata, eg due unclean shutdown on non-softdep filesystems.

split "slow (successful|failed) sync" message into mins/sec

shush shellcheck

fix previous

support -w to warn if the sync was too slow

update known_hosts

resolve relative paths

more quietness, from miod

update manpage to match script changes

add some syslog