Add unix domain socket support for Wnn backend and use it by
default. If $[CJKT]SERVER environment variable is defined, the
value is used as the server host name. In JSERVER case, for
example, the server name is determined in the following way:
$JSERVER --(if null)--> wnn-jserver --(if null)--> "unix"
If the server host name is null or matches "^unix$", unix
domain socket is used. The path name of the socket can be
specified in wnn-[cjkt]udpath custom variable.