MFC r269669:

 Split up sys_ktimer_getoverrun() into a sys_ and a kern_ variant
 and export the kern_ version needed by an upcoming linuxolator change.

 Sponsored by:	DARPA,AFRL
MFC r269583:

 Provide -o vers= support for mount_nfs.

 Our mount_nfs does use -o nfsv<2|3|4> or -2 or -3 to specify the version.
 OSX (these days), Solaris, and Linux use -o vers=<2,3,4>.

 With the upcoming autofs support we can make a lot of (entrerprisy) setups
 getting mount options from LDAP just work by providing -o vers= compatibility.

 Reviewed by:	wblock, bjk (man page), rmacklem, emaste
 Sponsored by:	DARPA,AFRL
PR:		192379
MFC r259884:

 Correct warnings comparing unsigned variables < 0 constantly reported
 while building kernels.  All instances removed are indeed unsigned so
 the expressions could not be true.
MFC r259885:

 As constantly reported during kernel compilation, m_buflen is unsigned so
 can never be < 0.  Remove the expression, which can never be true.
MFC r259916:

 Use feature_present(3) to determine whether to open an INET or an
 INET6 socket when needed to allow pfctl to work on noinet and noinet6
 kernels (and try to provide a fallback using AF_LINK as best effort).
 Adjust the Makefile to also respect relevant src.conf(5) options
 for compile time decisions on INET and INET6 support.

 Reviewed by:	glebius (no objections)