device file is closed, rather on every descriptor close. Instead of
getting the exclusive lock on open and releasing on close, get
the lock only when needed in sysmonioctl_envsys().
Fixes kern/14368 by Anthony Mallet. Okayed by Jason Thorpe.
While here, also inline sysmon_envsys_init() to sysmonopen_envsys(),
and g/c sysmonioctl_wdog() prototype.
57b53bc9a8