Add -share -noshare command line option to plink to share SSL connections.

doc/man-pl.but

@@ -182,6 +182,15 @@ which of the agent's keys to use. }
 \dd Allow use of an authentication agent. (This option is only necessary
 to override a setting in a saved session.)
 
+\dt \cw{\-noshare}
+
+\dd Don't test and try to share an existing connection, always make
+a new connection.
+
+\dt \cw{\-share}
+
+\dd Test and try to share an existing connection.
+
 \dt \cw{\-hostkey} \e{key}
 
 \dd Specify an acceptable host public key. This option may be specified

doc/plink.but

@@ -75,6 +75,8 @@ use Plink:
 \c   -i key    private key file for user authentication
 \c   -noagent  disable use of Pageant
 \c   -agent    enable use of Pageant
+\c   -noshare  disable use of connection sharing
+\c   -share    enable use of connection sharing
 \c   -hostkey aa:bb:cc:...
 \c             manually specify a host key (may be repeated)
 \c   -m file   read remote command(s) from file
@@ -237,6 +239,28 @@ line.
 
 (This option is only meaningful with the SSH-2 protocol.)
 
+\S2{plink-option-share} \I{-share-plink}\c{-share}:
+Test and try to share an existing connection.
+
+This option tris to detect if an existing connection can be shared
+(See \k{config-ssh-sharing} for more information about SSH connection
+sharing.) and reuses that connection.
+
+A Plink invocation of the form:
+
+\c plink -share <session>
+\e              iiiiiiiii
+
+will test whether there is currently a viable \q{upstream} for the
+session in question, which can be specified using any syntax you'd
+normally use with Plink to make an actual connection (a host/port
+number, a bare saved session name, \c{-load}, etc). If no \q{upstream}
+viable session is found and \c{-share} is specified, this connection
+will be become the \q{upstream} connection for subsequent connection
+sharing tries.
+
+(This option is only meaningful with the SSH-2 protocol.)
+
 \S2{plink-option-shareexists} \I{-shareexists-plink}\c{-shareexists}:
 test for connection-sharing upstream