datatype: add define for maximum number of bits and mask of datatype id

datatype: add define for maximum number of bits and mask of datatype id

The id of concat datatypes is composed of the ids of the individual
datatypes. Add a define for the number of bits for each datatype id
and a mask.

The number of bits is chosen as 6, allowing for 63 datatypes, or twice
as much as we currently have. This allows for concatenations of 5
types using 32 bits.

The value is statically chosen instead of basing it on the current
numbers of datatypes since we don't want the maximum concatenation
size to vary between versions, also new versions are supposed to be
able to propery parse a ruleset generated by an older version.

Signed-off-by: Patrick McHardy <kaber@trash.net>