libsys: add a libsys.h

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