libsys: add a libsys.h

This declares an API for libsys which currently consists of
__sys_<foo>() declarations for system call stubs and function pointer
typedefs of the form __sys_<foo>_t. The vast majority of the
implementation resides in a generated _libsys.h which ensures that all
system call stub declarations match syscalls.master.

Reviewed by: kib
Differential Revision: https://reviews.freebsd.org/D44387

libsys: generate private symbol map

We now export all _ and __sys_ prefixed syscalls stubs from libc and
libsys so that libsys can replace them.

Reviewed by: kib, emaste, imp
Pull Request: https://github.com/freebsd/freebsd-src/pull/908

makesyscalls: don't make syscall.mk by default

We only want to produce syscall.mk for the main syscall table so default
to not producing it (send it to /dev/null) and add a syscalls.conf to
sys/kern to trigger the creation of sys/sys/syscall.mk. This eliminates
the need for entries in other syscalls.conf files and is a cleaner
pattern going forward.

Reviewed by: kevans, imp
Differential Revision: https://reviews.freebsd.org/D42663