hush: first stab at function support. argv passing is not coded yet.

hush: first stab at function support. argv passing is not coded yet.
 Only very rudimentary testing was done.
 With function support off, code growth is zero, with it on:

function                                             old     new   delta
run_list                                            2158    2339    +181
parse_stream                                        1929    2044    +115
find_builtin                                          24      67     +43
find_function                                          -      36     +36
file_get                                             244     264     +20
pseudo_exec_argv                                     145     160     +15
free_strings                                           -       7      +7
free_pipe                                            183     181      -2
done_word                                            735     728      -7
expand_variables                                    2227    2204     -23
------------------------------------------------------------------------------
(add/remove: 2/0 grow/shrink: 5/3 up/down: 417/-32)           Total: 385 bytes