Allow directives to have no parameters in the parser, and check for it at

Allow directives to have no parameters in the parser, and check for it at
a lower level.

To reduce code duplication, refactor several of the large if/else structures
into multiple functions with a lookup table.

Add support for registers to be used in directive expressions.

Allow directives to have underscores and numbers (still cannot start with
numbers).

svn path=/trunk/yasm/; revision=1757