Allow server to bind an OS-selected UDP port with -p 0.

It looks like Mosh used to be able to do this but the capability was
lost when port ranges were added in 141ec23.

man/mosh-server.1

@@ -70,7 +70,8 @@ IP address of the local interface to bind (for multihomed hosts)
 
 .TP
 .B \-p \fIPORT\fP[:\fIPORT2\fP]
-UDP port number or port-range to bind
+UDP port number or port-range to bind.  \fB\-p 0\fP will let the
+operating system pick an available UDP port.
 
 .TP
 .B \-c \fICOLORS\fP

man/mosh.1

@@ -183,9 +183,12 @@ Synonym for \-\-family=inet6
 Use a particular server-side UDP port or port range,
 for example, if this is the
 only port that is forwarded through a firewall to the
-server. Otherwise, \fBmosh\fP will choose a port between 60000 and
+server. With \fB\-p 0\fP, the server will let the operating system pick an
+available UDP port. Otherwise, \fBmosh\fP will choose a port between 60000 and
 61000.
 
+
+
 .TP
 .B \-\-bind\-server={ssh|any|\fIIP\fP}
 Control the IP address that the \fBmosh-server\fP binds to.

scripts/mosh.pl

@@ -187,13 +187,16 @@ if ( defined $port_request ) {
   if ( $port_request =~ m{^(\d+)(:(\d+))?$} ) {
     my ( $low, $clause, $high ) = ( $1, $2, $3 );
     # good port or port-range
-    if ( $low <= 0 or $low > 65535 ) {
-      die "$0: Server-side (low) port ($low) must be within valid range [1..65535].\n";
+    if ( $low < 0 or $low > 65535 ) {
+      die "$0: Server-side (low) port ($low) must be within valid range [0..65535].\n";
     }
     if ( defined $high ) {
       if ( $high <= 0 or $high > 65535 ) {
 	die "$0: Server-side high port ($high) must be within valid range [1..65535].\n";
       }
+      if ( $low == 0 ) {
+	die "$0: Server-side port ranges may not be used with starting port 0 ($port_request).\n";
+      }
       if ( $low > $high ) {
 	die "$0: Server-side port range ($port_request): low port greater than high port.\n";
       }

src/network/network.cc

@@ -261,8 +261,8 @@ Connection::Connection( const char *desired_ip, const char *desired_port ) /* se
      try INADDR_ANY. If a port request is given, we bind only to that port. */
 
   /* convert port numbers */
-  int desired_port_low = 0;
-  int desired_port_high = 0;
+  int desired_port_low = -1;
+  int desired_port_high = -1;
 
   if ( desired_port && !parse_portrange( desired_port, desired_port_low, desired_port_high ) ) {
     throw NetworkException("Invalid port range", 0);
@@ -307,10 +307,10 @@ bool Connection::try_bind( const char *addr, int port_low, int port_high )
 
   int search_low = PORT_RANGE_LOW, search_high = PORT_RANGE_HIGH;
 
-  if ( port_low != 0 ) { /* low port preference */
+  if ( port_low != -1 ) { /* low port preference */
     search_low = port_low;
   }
-  if ( port_high != 0 ) { /* high port preference */
+  if ( port_high != -1 ) { /* high port preference */
     search_high = port_high;
   }
 
@@ -688,7 +688,6 @@ bool Connection::parse_portrange( const char * desired_port, int & desired_port_
     desired_port_high = desired_port_low;
     return true;
   }
-
   /* port range; parse high port */
   const char * cp = end + 1;
   errno = 0;
@@ -708,5 +707,11 @@ bool Connection::parse_portrange( const char * desired_port, int & desired_port_
     return false;
   }
 
+  if ( desired_port_low == 0 ) {
+    fprintf( stderr, "Low port 0 incompatible with port ranges\n" );
+    return false;
+  }
+
+
   return true;
 }