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/exec.go

@@ -1,6 +1,18 @@
 package os
 
+import "syscall"
+
 type Signal interface {
 	String() string
 	Signal() // to distinguish from other Stringers
 }
+
+// Getpid returns the process id of the caller, or -1 if unavailable.
+func Getpid() int {
+	return syscall.Getpid()
+}
+
+// Getppid returns the process id of the caller's parent, or -1 if unavailable.
+func Getppid() int {
+	return syscall.Getppid()
+}