hush: make var nesting code independent of "local" support

hush: make var nesting code independent of "local" support

Also, add code to abort at ~65000 function recursion depth.
SEGVing is not as nice as exiting with a message (and restoring termios!):

$ f() { echo -n .; f; }; f
....<many dots later>....hush: fatal recursion (depth 65281)

function                                             old     new   delta
run_pipe                                            1826    1890     +64
pseudo_exec_argv                                     544     554     +10
parse_and_run_file                                    71      80      +9
i_getch                                              104     107      +3
done_command                                          99     102      +3
set_local_var                                        508     510      +2
helper_export_local                                  214     215      +1
builtin_local                                         49      46      -3
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 7/1 up/down: 92/-3)              Total: 89 bytes

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>