Move find_toplevel_char to utils.[ch]

find_toplevel_char is being used more and more outside of linespec.c, so
this patch moves it into utils.[ch].

gdb/ChangeLog:

	* linespec.c (find_toplevel_char): Moved to ...
	* utils.c (find_toplevel_char): ... here.
	* linespec.h (find_toplevel_char): Moved to ...
	* utils.h (find_toplevel_char): ... here.