[asterisk-dev] fix for DAHTOOL-78: race: both init script and hotplug may configure astribanks at boot time

[Tzafrir Cohen]

The following two patches (The first adds a helper functoinality and the
second includes the fix) fix a race in case a system uses hot-pluggable
DAHDI and yet includes the iinit script.

IIt doesn't cover some edge cases, but those are much less interesting
(systems where not all the spans are configured).