Allow running zoneserver under systemd

[pillarsdotnet]

When invoking zoneserver directly from systemd, the PGID is
already set to the PID of the main zoneserver process, and
attempting to call setpgid(0,0) results in an ENOPERM error.

[samboy]

According to the getpgid() manual page:

getpgid() returns the PGID of the process specified by pid. If pid is zero, the process ID of the calling process is used. (Retrieving the PGID of a process other than the caller is rarely necessary, and the POSIX.1 getpgrp() is preferred for that task.) <<

That in mind, I think it would be better to use getpgrp() instead of getpgid(0). Let’s avoid Linuxisms in Mara’s code.

[samboy]

Let’s look at this code:

#include <stdio.h>
#include <sys/types.h>
#include <unistd.h>

int main() {
        printf("getpgid(0) %d\n",getpgid(0));
        printf("getpgrp() %d\n",getpgrp());
        printf("getpid() %d\n",getpid());
}

This shows that getpgid(0) returns the same value as getpgrp().

Rejecting merge request: Do things the POSIX way whenever possible, and avoid Linuxisms.

[samboy]

Original patch:

if(getpgid(0) != getpid() && setpgid(0,0)) {

The patch I actually applied:

if(setpgid(0,0) && getpgrp() != getpid()) {

I have released MaraDNS 3.5.0019 with this change (and, in addition, another example coLunacyDNS script: Give coLunacyDNS 10.1.2.3.ip4.internal., and coLunacyDNS gives you 10.1.2.3 back; this script works for all IPv4 IPs).