lib: parser: add named variables in CLI

lib: parser: add named variables in CLI

struct cmd_token now has a "varname" field which is derived from the
DEFUN's string definition.  It can be manually specified with "$name"
after some token, e.g. "foo WORD$var".  A later commit adds code to
automatically fill the value if nothing is specified.

Signed-off-by: David Lamparter <equinox@opensourcerouting.org>