remove support for syscall(2) -- the "indirection system call" because

remove support for syscall(2) -- the "indirection system call" because
it is a dangerous alternative entry point for all system calls, and thus
incompatible with the precision system call entry point scheme we are
heading towards.  This has been a 3-year mission:
First perl needed a code-generated wrapper to fake syscall(2) as a giant
switch table, then all the ports were cleaned with relatively minor fixes,
except for "go".  "go" required two fixes -- 1) a framework issue with
old library versions, and 2) like perl, a fake syscall(2) wrapper to
handle ioctl(2) and sysctl(2) because "syscall(SYS_ioctl" occurs all over
the place in the "go" ecosystem because the "go developers" are plan9-loving
unix-hating folk who tried to build an ecosystem without allowing "ioctl".
ok kettenis, jsing, afresh1, sthen