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.