This is almost always true already; fix up the few non-standard ones.
This is enforced with an assert, and I ran the entire test suite to
double-check.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Any file in this directory which is executable and whose name only
consists of alphanumeric characters, space, '.', '-' or '_' will be
automatically loaded when lightningd starts (unless suppressed with
commandline options).