defs: rework CONF_DIRS_NULSTR() macro

defs: rework CONF_DIRS_NULSTR() macro

The macro is generically useful for putting together search paths, hence
let's make it truly generic, by dropping the implicit ".d" appending it
does, and leave that to the caller. Also rename it from
CONF_DIRS_NULSTR() to CONF_PATHS_NULSTR(), since it's not strictly about
dirs that way, but any kind of file system path.

Also, mark CONF_DIR_SPLIT_USR() as internal macro by renaming it to
_CONF_PATHS_SPLIT_USR() so that the leading underscore indicates that
it's internal.