Move e_pid and e_pgid to libprocess, Add get_self_pid () syscall

ce/mprintf.c

@@ -1,7 +1,7 @@
 #include "mprintf.h"
-#include "self.h"
 #include <malloc.h>
 #include <printf.h>
+#include <process_self.h>
 #include <stdarg.h>
 #include <string.h>
 #include <system.h>
@@ -22,6 +22,6 @@ void mprintf (const char* fmt, ...) {
 
   va_end (args);
 
-  mail_send (e_pgid, buf, len);
+  mail_send (process_get_exec_pgid (), buf, len);
   free (buf);
 }