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/.gitignore

@@ -15,6 +15,7 @@
 /printhdr
 /progress
 /rsync
+/runbg
 /str
 /strmatch
 /systemf

test/Makefile.am

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

test/runbg.c

@@ -0,0 +1,23 @@
+#include "check.h"
+
+#define file "/tmp/runbg.txt"
+
+int main(void)
+{
+	char *cmd[] = {
+		"sh", "-c", "echo 'hello world' > " file, NULL
+	};
+
+	erase(file);
+	test(runbg(cmd, 200000), "Calling runbg");
+
+	if (test(fexist(file), "Verifying %s does not yet exist", file))
+		return 1;
+
+	test(usleep(300000), "Waiting for runbg");
+
+	if (test(!fexist(file), "Verifying %s has been created", file))
+		return 1;
+
+	return 0;
+}