Replace header_files global by per-objfile field.

	Replace header_files global by per-objfile field.
	* gdb-stabs.h (struct dbx_symfile_info):  Add fields header_files,
	n_header_files, n_allocated_header_files.
	* stabsread.h (header_files, n_header_files, n_allocated_header_files):
	Replace externs by macros HEADER_FILES, N_HEADER_FILES, and
	N_ALLOCATED_HEADER_FILES.
	* dbxread.c (dbx_symfile_finish):  Free HEADER_FILES.
	(free_header_files, init_header-files):  Don't free/init headerfiles.
	(various functions):  Use macros instead of header_files globals.
	* stabsread.c (various functions):  Likewise.