ddb: reliably fail with ambiguous commands

db_cmd_match had an even/odd bug, where if a third command was partially
matched (or any odd number greater than one) the search result would be
set back from CMD_AMBIGUOUS to CMD_FOUND, causing the last command in
the list to be executed instead of failing the match.

Reported by: mlaier
Reviewed by: markj, mlaier, vangyzen
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D28659