luci-base: validation.js: optionally support negative prefixes

Support negative prefix length in the `cidr`, `cidr4`, `cidr6`, `ipmask`,
`ipmask4` and `ipmask6` data types when an optional truish flag is passed
to the datatype name.

Ref: #4812
Signed-off-by: Jo-Philipp Wich <jo@mein.io>