stand: separate the command lookup from the command execution

Factor out interp_lookup_cmd to search for a command from
interp_builtin_cmd. This simplifies the latter and can be used to expand
lua to ask if a command exists.

Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D36363

(cherry picked from commit 113dfadd5c8c2b8a566bf4d0e969e1dff62c9e2f)