Collapse successive char deletions in the undo log

Collapse successive char deletions in the undo log

* src/cmds.c (remove_excessive_undo_boundaries): New function,
extracted from Fself_insert_command.
(Fdelete_char, Fself_insert_command): Use it.
* src/fileio.c (Fmake_symbolic_link): Rename arg to `target'.
* src/keyboard.c (syms_of_keyboard): `top-level' shouldn't be special.