mxmount: Add option --reexport-only

Calling mxmount with --reexport-only will not try to mount filesystems,
but just rewrite /etc/exports and call exportfs -ra.

This is supposed to be done by clusterd, when the export groups in
/etc/hostconfig were changed.

mxmount/mxmount

@@ -3,6 +3,7 @@
 use strict;
 use warnings;
 use Sys::Hostname;
+use Getopt::Long;
 
 my $configfile = "/etc/mxmounts";
 my $exports    = "/etc/exports";
@@ -16,6 +17,13 @@ my @exports;
 my %V;
 my %D;
 
+our $USAGE=<<"_EOF_";
+usage: $0
+       $0 --reexport-only
+_EOF_
+
+our ($opt_reexport_only);
+
 $fullhostname = hostname();
 ($hostname) = $fullhostname =~ /^(.*?)\./;
 
@@ -40,9 +48,15 @@ foreach(@lines) {
 }
 
 add_data0_if_not_present();
-mount_all();
-create_exports();
 
+my %options;
+GetOptions (
+    'reexport-only' => \$opt_reexport_only,
+) or die $USAGE;
+@ARGV and die $USAGE;
+
+mount_all() unless $opt_reexport_only;
+create_exports();
 system("exportfs -ra");
 
 sub safe_qx { open my $pipe,'-|',@_; return join('',<$pipe>) }