A netmask that contains 1-bits after 0-bits (the 1-bits are not
contiguous on the left side) is invalid [1] [2].

The code before this PR used to parse and accept such
non-left-contiguous netmasks. However, a coming change that will alter
`CNetAddr::ip` to have flexible size would make juggling with such
netmasks more difficult, thus drop support for those.

[1] https://en.wikipedia.org/wiki/Classless_Inter-Domain_Routing#Subnet_masks
[2] https://tools.ietf.org/html/rfc4632#section-5.1

ninja all check-all