Factor out the back ends' plug log functions.

I'm about to want to make a change to all those functions at once, and
since they're almost identical, it seemed easiest to pull them out
into a common helper. The new source file be_misc.c is intended to
contain helper code common to all network back ends (crypto and
non-crypto, in particular), and initially it contains a
backend_socket_log() function which is the common part of ssh_log(),
telnet_log(), rlogin_log() etc.

Recipe

@@ -233,7 +233,7 @@ SFTP     = sftp int64 logging
 
 # Miscellaneous objects appearing in all the network utilities (not
 # Pageant or PuTTYgen).
-MISC     = timing callback misc version settings tree234 proxy conf
+MISC     = timing callback misc version settings tree234 proxy conf be_misc
 WINMISC  = MISC winstore winnet winhandl cmdline windefs winmisc winproxy
          + wintime winhsock errsock
 UXMISC   = MISC uxstore uxsel uxnet uxpeer cmdline uxmisc uxproxy time