Allocate sufficient space for string buffer

Allocate sufficient space for string buffer

	* input-scrub.c (input_scrub_include_sb): Use sb_build to
	allocate sufficient space for from_sb.  Use sb_terminate to
	terminate string.
	* read.c (read_a_source_file): Use sb_build to allocate
	sufficient space and replace sb_add_string with sb_add_buffer.
	(s_macro): Likewise.
	(input_scrub_insert_line): Likewise.
	(s_irp): Use sb_build to allocate sufficient space.
	(do_repeat): Use sb_build to allocate sufficient space
	for many.
	* sb.c (sb_build): Remove static.
	* sb.h (sb_build): New prototype.