Move syscall definitions to an external package.

[skx]

Rather than just defining the few syscalls we use, inline, in stdlib.lisp we now auto-generate all known system call numbers.

For example:

 $ head -n 20 packages/syscall.lisp 
;;; syscall-constants
;;
;; These constants define known syscalls, and are auto-generated.
;;
(defconst SYS_read 0)
(defconst SYS_write 1)
(defconst SYS_open 2)
(defconst SYS_close 3)
(defconst SYS_stat 4)
(defconst SYS_fstat 5)
(defconst SYS_lstat 6)
(defconst SYS_poll 7)
(defconst SYS_lseek 8)
(defconst SYS_mmap 9)
(defconst SYS_mprotect 10)
(defconst SYS_munmap 11)
(defconst SYS_brk 12)
(defconst SYS_rt_sigaction 13)
(defconst SYS_rt_sigprocmask 14)
(defconst SYS_rt_sigreturn 15)

This means they're available for use, directly, by users and don't need to be defined specially.

This closes #281.