lib: Fix nondeterministic command matches in rare cases

lib: Fix nondeterministic command matches in rare cases

When a user erroneously defines two commands which can
match the same input and at least one of the tokens defined
last in the command is a selector or option, the matcher
does not detect an ambiguous match and matches the command
installed first (leftmost in the graph).

Fix is to do a full walkthrough of the follow set when
matching the final token in a command to check that there
is exactly one possible match, and to throw an ambiguity
error otherwise.

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