syscalls.master: correct return type of {read,write}v

This was missed when read/write, etc were updated to return ssize_t.

Fixes: 2e83b2816183 Fix a few syscall arguments to use size_t instead of u_int.

Reviewed by: imp, kib
Differential Revision: https://reviews.freebsd.org/D44930

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