stand: Add interp_has_builtin_cmd to see if we have a command
interp_has_builtin_cmd() will try to lookup the passed in command and
returns true if it was found, false otherwise.
Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D36364
(cherry picked from commit a5948d40ad060140bf5b995f5409458a18ced0ce)