Mark various removed system calls as OBSOL instead of UNIMPL.

This is mostly a cosmetic change except that obsolete system calls are
assigned meaningful names in the names arrays which means that using
tools like kdump or truss against binaries invoking these system calls
will print out the name instead of the number. The script I use to
generate the XML list of syscalls for GDB also ignores UNIMPL but not
OBSOL entries. In general UNIMPL should only be used to reserve
placeholders for system calls that have never been implemented while
system calls that existed at one time in FreeBSD but were removed
should be marked OBSOL instead.

Reviewed by: brooks, kib, imp
Approved by: re (gjb)
Differential Revision: https://reviews.freebsd.org/D17344