PR gas/827

	PR gas/827
	* as.h (rs_dummy): Define.
	* symbols.c (symbol_X_add_number): New function.
	* symbols.h (symbol_X_add_number): Declare.
	* stabs.c (aout_process_stab): Tidy symbol frag setting.
	* read.c (assign_symbol): New function, split out from s_set.
	Use symbol_find_or_make.  Leave fr_type of dummy frag as rs_dummy.
	Fix COFF hacks for multi-emulation gas.
	(s_set): Call assign_symbol.  Remove "register" keyword.
	(set_zero_frag): New function.
	(pseudo_set): Always check for assignment to section syms.
	Always set segment and frag of symbol, and likewise extern for
	aout/bout.  Handle assignment of sym=sym+/-const specially.  Don't
	special case exp.X_add_number non-zero for O_symbol expressions.
	(equals): Use assign_symbol.