sysmon(9): Fix callout/thread synchronization.

Callout may ONLY take sme_work_mtx, at IPL_SOFTCLOCK; MUST NOT touch
sme_mtx at IPL_NONE.  All state the callout needs is serialized by
sme_work_mtx now:

- calls to sme_schedule_callout
- calls to sme_schedule_halt
- struct sysmon_envsys::sme_events_timeout
- struct sysmon_envsys::sme_events_list
- struct sysmon_envsys::sme_callout_state
- struct envsys_data::flags
  => yes, this is a little silly -- used for ENVSYS_FNEED_REFRESH
  => should maybe separate the static driver-defined features from
    the state flags needed by sysmon_envsys but not important now

Sleeping under sme_work_mtx (except on other adaptive locks at
IPL_SOFTCLOCK) is forbidden.  Calling out to the driver under
sme_work_mtx is forbidden.

This should properly fix:

https://mail-index.netbsd.org/tech-kern/2015/10/14/msg019511.html
PR kern/56592

(riastradh)