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/os/file.go

@@ -196,8 +196,3 @@ func Readlink(name string) (string, error) {
 func TempDir() string {
 	return "/tmp"
 }
-
-// Getpid is a stub (for now), always returning 1
-func Getpid() int {
-	return 1
-}