Module API for Forking

* create module API for forking child processes.
* refactor duplicate code around creating and tracking forks by AOF and RDB.
* child processes listen to SIGUSR1 and dies exitFromChild in order to
  eliminate a valgrind warning of unhandled signal.
* note that BGSAVE error reply has changed.

valgrind error is:
  Process terminating with default action of signal 10 (SIGUSR1)

tests/unit/moduleapi/fork.tcl

@@ -0,0 +1,32 @@
+set testmodule [file normalize tests/modules/fork.so]
+
+proc count_log_message {pattern} {
+    set result [exec grep -c $pattern < [srv 0 stdout]]
+}
+
+start_server {tags {"modules"}} {
+    r module load $testmodule
+
+    test {Module fork} {
+        # the argument to fork.create is the exitcode on termination
+        r fork.create 3
+        wait_for_condition 20 100 {
+            [r fork.exitcode] != -1
+        } else {
+            fail "fork didn't terminate"
+        }
+        r fork.exitcode
+    } {3}
+
+    test {Module fork kill} {
+        r fork.create 3
+        after 20
+        r fork.kill
+        after 100
+
+        assert {[count_log_message "fork child started"] eq "2"}
+        assert {[count_log_message "Received SIGUSR1 in child"] eq "1"}
+        assert {[count_log_message "fork child exiting"] eq "1"}
+    }
+
+}