ddb: reliably fail with ambiguous commands

[rlibby]

Summary

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.

Test Plan

# sysctl debug.kdb.enter=1
debug.kdb.enter:KDB: enter: sysctl debug.kdb.enter
 [ thread pid 895 tid 100225 ]
Stopped at      kdb_sysctl_enter+0x95:  movq    $0,0x128d790(%rip)
db> show a 
Ambiguous
db> show b
Ambiguous
db> show c
Ambiguous
db> show d
Ambiguous
db> show f
Ambiguous
db> show i
Ambiguous