Turn off caching when reading network manager config files, as they may be modified by the nm cli command

https://forum.virtualmin.com/t/virtualmin-virtualserver-ip-copy-to-services/133442/12

net/nm-lib.pl

@@ -619,6 +619,7 @@ foreach my $iface (@fix) {
 sub read_nm_config
 {
 my ($f) = @_;
+&unflush_file_lines($f);	# NM may have modified this file
 my $lref = &read_file_lines($f, 1);
 my @rv;
 my $sect;