Massive update of directive handling, including handling of extern, global,

Massive update of directive handling, including handling of extern, global,
and common declare.  The latter no longer passes through objfmt at parse time;
instead the objfmt must handle them at output time (objfmt-specific
extensions are parsed & stored by the parser).  Directives are now handled
using a list (with function pointers) rather than a single function entry
point.

svn path=/trunk/yasm/; revision=1819