hush: shrink and make more versatile error-reporting machinery

hush: shrink and make more versatile error-reporting machinery

function                                             old     new   delta
syntax_error_unterm_ch                                 -      31     +31
syntax_error_unterm_str                                -      14     +14
parse_stream                                        2356    2361      +5
syntax_error_at                                       12      14      +2
syntax_error                                          25      27      +2
syntax_error_unterminated                             28       -     -28
expand_variables                                    2063    2031     -32
------------------------------------------------------------------------------
(add/remove: 2/1 grow/shrink: 3/1 up/down: 54/-60)             Total: -6 bytes
   text    data     bss     dec     hex filename
  67278     197    3184   70659   11403 busybox_old
  67228     197    3184   70609   113d1 busybox_unstripped