Fix two problems in CSubNet parsing

Fix two CSubNet constructor problems:

• The use of /x where 8 does not divide x was broken, due to a bit-order issue

• The use of e.g. 1.2.3.4/24 where the netmasked bits in the network are not 0 was broken. Fix this by explicitly normalizing the netwok according to the bitmask.

Also add tests for these cases.

Fixes #6179. Thanks to @jonasschnelli for reporting and initial fix.