adblock: reduce memory consumption

adblock: reduce memory consumption

* rework shallalist processing: significantly reduce memory consumption
during archive extraction and merging.
* considerable reduce memory consumption during adblock source
processing.
* considerable reduce memory consumption of sort (sorts only the domain
list and not the bigger dnsmasq file)

other changes:
* auto detection/defaults for adb_if, adb_dev, adb_ntpsrv, adb_maxloop,
adb_maxtime and adb_minspace - these options can be safely removed from
previous adblock configuration file
* check total memory and main uhttpd configuration on startup
* documentation update

Signed-off-by: Dirk Brenken <dirk@brenken.org>