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