STABS debugging information. This includes, naturally, several draft

STABS debugging information.  This includes, naturally, several draft
changes to the dbgfmt interface, and other assorted updates, including:
* yasm.c now calls df->initialize() and df->generate()
* a dbgfmt bytecode type with associated handling
* yasm_output_reloc_func type for use particularly by dbgfmts
* df: initialize updated; generate, bc_dbgfmt_data_{output|delete|print} added
* null-dbgfmt structure brought in line with these additions
* elf-objfmt made aware of stabs sections, and what to do with them

The bad news:
* just enough stabs output to support line number information in GDB
* GDB identifies function labels off by 3 bytes in my test, but line
  numbers remain correct, somehow.  Unknown whether stabs-dbgfmt or GDB
  at fault.

svn path=/trunk/yasm/; revision=1037