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

libsystem/system.c

@@ -104,3 +104,5 @@ int exec_partial (const char* volume, const char* path) {
 }
 
 int exec_partial_fini (int pid) { return (int)do_syscall (SYS_EXEC_PARTIAL_FINI, pid); }
+
+int get_self_pid (void) { return (int)do_syscall (SYS_GET_SELF_PID, 0); }

libsystem/system.h

@@ -108,4 +108,7 @@ int exec_partial (const char* volume, const char* path);
 /* Finish process for execution - run it! */
 int exec_partial_fini (int pid);
 
+/* get this process' PID */
+int get_self_pid (void);
+
 #endif // _LIBMSL_M_SYSTEM_H