modprobe: add -s/--syslog flag

There is a desire to ensure that modprobe as a service can log to
kmesg to help triage issues, so add support for the -s or --syslog
flag to do so.

SideEffects:
- help goes to stdout instead of stderr.
- verbose flag once, sets DEBUG, twice, sets VERBOSE minimum.
- quiet flag sets WARNING minimum.

Bug: 159424228
Bug: 151950334
Test: use modprobe as a service to load modules, check logs
Change-Id: I884995f364b0fc604861797eb90d7225a372f864