MFC r269669,271743:

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

 Implement most of timer_{create,settime,gettime,getoverrun,delete}
 for amd64/linux32.  Fix the entirely bogus (untested) version from
 r161310 for i386/linux using the same shared code in compat/linux.

 Reviewed by:		jhb
 Differential Revision:	D784
Sponsored by:	DARPA/AFRL
 This is a direct commit rather than an MFC of r271744.

   Re-gen after r272029 (r271743 in head) implementing most of
   timer_{create,settime,gettime,getoverrun,delete}.

Sponsored by:	DARPA/AFRL