MFC r219405:

MFC r219405:
Extend struct sysvec with new method sv_schedtail, which is used for an
explicit process at fork trampoline path instead of eventhadler(schedtail)
invocation for each child process.

Remove eventhandler(schedtail) code and change linux ABI to use newly added
sysvec method.

While here replace explicit comparing of module sysentvec structure with the
newly created process sysentvec to detect the linux ABI.

MFC r219421:
Remove bogus since r219405 check of the Linux ABI.

git-svn-id: svn://svn.freebsd.org/base/stable/8@222846 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f