gas read_a_source_file #APP processing

This fixes some horrible code using do_scrub_chars.  What we had ran
text through do_scrub_chars twice, directly in read_a_source_file and
again via the input_scrub_include_sb call.  That's silly, and since
do_scrub_chars is a state machine, possibly wrong.  More silliness is
evident in the temporary malloc'd buffer for do_scrub_chars output,
which should have been written directly to sbuf.

So, get rid of the do_scrub_chars call and support functions, leaving
scrubbing to input_scrub_include_sb.  I did wonder about #NO_APP
overlapping input_scrub_next_buffer buffers, but that should only
happen if the string starts in one file and finishes in another.

	* read.c (scrub_string, scrub_string_end): Delete.
	(scrub_from_string): Delete.
	(read_a_source_file): Rewrite #APP processing.