net80211: migrate if_flags, if_drvflags out of most source files

Migrate both if_flags and if_drvflags out of most source files.
Ideally it'd only be referenced in ieee80211_freebsd.c, but for now
it also ignores references in ieee80211_ioctl.c.

• migrate if_flags set to if_setflags
• migrate if_flags get to if_getflags
• migrate if_drvflags get to if_getdrvflags
• add ieee80211_vap_ifp_check_is_monitor() and ieee8021_vap_ifp_check_is_simplex() to abstract out the IFF_MONITOR and IFF_SIMPLEX flag checks.
• add ieee80211_vap_ifp_check_is_running() and ieee80211_vap_ifp_set_running_state() to represent what IFF_DRV_RUNNING means (ie, mark the underlying OS network interface as active and inactive.)

Notably this doesn't yet clear up OACTIVE; I need to better describe
that.

Differential Revision: https://reviews.freebsd.org/D50405
Reviewed by: bz