Simplify saved_command_line handling

Simplify saved_command_line handling

There doesn't seem to be much point in trying to reuse this buffer.
Prefer simplicity instead.

(In case you're wondering whether this fixes an off-by-one: linelength
is misnamed; it's really a size including terminating null char.)

gdb/ChangeLog:
2016-03-09  Pedro Alves  <palves@redhat.com>

	* event-top.c (command_line_handler): Use xfree + xstrdup instead
	of xrealloc + strcpy.
	* main.c (captured_main): Use xstrdup instead of xmalloc plus
	manual clear.
	* top.c (saved_command_line): Rewrite comment.
	(saved_command_line_size): Delete.
	(command_line_input): Use xfree + xstrdup instead of xrealloc +
	strcpy.
	* top.h (saved_command_line_size): Delete declaration.