lib: make CLI completions less surprising

lib: make CLI completions less surprising

* If a token matches exactly at the end of input, it still
  shows up in completions, e.g.

      ex# clear<?>
	clear  Reset functions
      ex(config)# ip route 1.2.3.4<?>
        A.B.C.D    IP destination prefix
        A.B.C.D/M  IP destination prefix (e.g. 10.0.0.0/8)

* If a token in mid-line exactly matches one token and
  partially matches on one or more additional tokens,
  the command tree(s) under the partially matching tokens
  will be ignored in favor of the exact match when compiling
  completions for the full line, e.g.

      ex(config)# ip <?>

  will only show completions for commands under 'ip' and not
  those under 'ipv6', which the input partially matches.

Signed-off-by: Quentin Young <qlyoung@cumulusnetworks.com>