runbg: new helper function inspired by the Infix project

The runbg() function is a delayed execvp() that has double-forked to the
background (to not create zombies).  It is useful for running commands
in the background for APIs that need to return a reply, e.g. a NETCONF
RPC to reboot the system.

Compared to the original in Infix, this version double forks to reparent
itself to PID 1 (init), closes all open files, including stdio.

Signed-off-by: Joachim Wiberg <troglobit@gmail.com>

test/Makefile.am

@@ -30,6 +30,7 @@ TESTS          += pidfile
 TESTS          += printhdr
 TESTS          += progress
 TESTS          += rsync
+TESTS          += runbg
 TESTS          += str
 TESTS          += strmatch
 TESTS          += systemf