mxmount: Expand host groups by hostconfig

When /etc/mxmounts uses the netgroup syntax to export to a list of
clients (usually @amd), expand the list using hostconfig to a list of
hosts. This is one step for us to get rid of NIS.

We could expand

    /filesystem @group(opts)

to

    /filesystem host1(opts) hosts2(opts) host3(opts) host4(opts)...

but because our opts are about 50 characters and we have about 540 hosts
on @amd, we save about 27000 characters per line by converting to

    /filesystem -opts host1 host2 host3...

Notes:

* `hostconfig --list` does not only accept single tag (=group) names,
but complex boolean expressions. We could export to @1 (=allways true,
any known host) or @testing&!desktop. It is suggested, not to use this
as a feature.

* We need some mechnism to trigger a regeneration of the exports by
mxmount when the groups are modified.

mxmount/mxmount

@@ -78,11 +78,18 @@ sub create_exports {
 
     foreach my $mp (@$allmp) {
         next if($mp->{noexport});
-        @CMD = ($mp->{mountpoint}, $mp->{exportopts});
-
-        print join " ", "$exports: ", @CMD, "\n";
-        print EXPORTS join " ", @CMD, "\n";
-
+        my ($mountpoint,$exportopts)=($mp->{mountpoint}, $mp->{exportopts});    # '/amd/theinternet/1','@amd(sync,rw,...)'
+        my ($hostspec,$optspec)=$exportopts=~/^([^(]+)(.*)/;                    # '@amd','(sync,rw,...)'
+        my ($opts)=$optspec=~/\((.*)\)/;                                        # 'sync,rw,...'
+        my $hosts='';
+        warn "export $mountpoint to $hostspec opts $opts\n";
+        if (my ($group) = $hostspec=~/^@(.+)/) {
+            $hosts=`hostconfig --list $group`;                      # expanded group
+            $hosts or warn "group $group is empty\n";
+        } else {
+            $hosts=$hostspec;                                       # single host
+        }
+        $hosts and printf EXPORTS "%s -%s %s\n",$mountpoint,$opts,$hosts;
     }
     close EXPORTS;
 }