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.

testdata/stdlib.go

@@ -5,6 +5,7 @@ import (
 	"math/rand"
 	"os"
 	"strings"
+	"syscall"
 )
 
 func main() {
@@ -13,6 +14,14 @@ func main() {
 	fmt.Println("stdout:", os.Stdout.Name())
 	fmt.Println("stderr:", os.Stderr.Name())
 
+	// Package syscall, this mostly checks whether the calls don't trigger an error.
+	syscall.Getuid()
+	syscall.Geteuid()
+	syscall.Getgid()
+	syscall.Getegid()
+	syscall.Getpid()
+	syscall.Getppid()
+
 	// package math/rand
 	fmt.Println("pseudorandom number:", rand.Int31())