Merge branch 't/gui_improvements'

Merge branch 't/gui_improvements'

Was cooking for more than one month.

	bf1831 string: Speed up xvasprintf().
	bcc083 string: Add discard feature for for_each_line().
	e3868d string: Simplify return value of for_each_line().
	d1f0f0 string: Simplify for_each_line().
	23b121 string: Clean up for_each_line() and related functions.
	6256ed string: Replace the for_each_line_modes enum by a bitmap.
	14e689 gui: Don't sleep before executing the status command.
	e90c6c gui: Speed up add_spaces().
	20e2c6 gui: Rename do_exit().
	ef0508 gui: Remove superfluous cmd_died.
	d7562b gui: Check stdin for readability.
	a44fa6 gui: Discard overlong input lines.

Conflicts:
	gui.c