os: implement process related functions

This commit implements various process related functions like
os.Getuid() and os.Getpid(). It also implements or improves this support
in the syscall package if it isn't available yet.

src/syscall/syscall_libc.go

@@ -62,10 +62,6 @@ func Kill(pid int, sig Signal) (err error) {
 	return ENOSYS // TODO
 }
 
-func Getpid() (pid int) {
-	panic("unimplemented: getpid") // TODO
-}
-
 func Getenv(key string) (value string, found bool) {
 	data := append([]byte(key), 0)
 	raw := libc_getenv(&data[0])