Fix #70 by allowing overrides on the default (usually ".text") section.

Fix #70 by allowing overrides on the default (usually ".text") section.
The fix for this rippled into a lot of places, and I'm starting to see some
opportunities for cleaning up some of the object and objfmt structures.

* objfmt.h (yasm_objfmt_add_default_section): Move from standalone function
into objfmt-specific function.
(yasm_objfmt_module): Remove default_section_name string, and add objfmt
specific add_default_section function.
* yasm.c (main): Use slightly updated parameters when calling.

* section.h (yasm_section_is_default, yasm_section_set_default): New.
* section.c (yasm_section): Add def member (for default section flag).
(yasm_section_is_default, yasm_section_set_default): Implement.

* xdf-objfmt.c, bin-objfmt.c, dbg-objfmt.c, coff-objfmt.c: Implement.
Usually this required refactoring the objfmt-specific section data creation
into a separate function that could be used by both section_switch() and
the new add_default_section() functions, and changing section_switch() to
update changes to the section data if section was new or previously just a
default section, instead of the previous behavior of warning if the section
was not new.

* objfmt.c: Delete (no longer needed).
* Makefile.inc, Makefile.flat, libyasm.vcproj
Makefile.dj: Update to reflect removal.

* xdf-overdef.asm, win32-overdef.asm, elf-overdef.asm: Test.

svn path=/trunk/yasm/; revision=1390