inconsistent syntax context for database parameters

As all database parameters are in the same parser rule they are valid in all database contexts (lease, host and config) and of course (even it seems silly) for all database types (memfile, mysql, postgresql and cassandra).

It seems the dhcp4_lexer.ll does not follow this rule so must be fixed.

Note this only change the parser error message: instead of accepting the parameter and eventually detect the misuse later the parameter is simply not recognized...