Revamp object format functional interface a bit. Now, only the output()

Revamp object format functional interface a bit.  Now, only the output()
function gets a FILE *, and it's the only function that can write to a file.
The object file is thus not opened until AFTER parsing and most error checking
is complete.  Necessitated adding a special case for the dbg object format
because it needs to output to the "object" (debug) file from essentially every
function.  Added a global (debug_file) to support this.

svn path=/trunk/yasm/; revision=476