scan_path(): Do not recurse into hidden directories by default

Paths that start with a period ('.') are considered hidden in the Unix world.
scan_path() should arguably not recurse into these directories by default.
This patch makes it so, and introduces the "scan-hidden-path" config variable
for overriding the new default and revert to the old behaviour (scanning _all_
directories, including hidden .directories).

Signed-off-by: Johan Herland <johan@herland.net>
Signed-off-by: Lars Hjemli <larsh@prediktor.no>