hush: print cd error to stderr; use fopen_or_warn in builtin_source;

hush: print cd error to stderr; use fopen_or_warn in builtin_source;
 prepare builtin_unset for function support
libbb: do not clear errno in fopen_or_warn

function                                             old     new   delta
builtin_unset                                        242     271     +29
fopen_or_warn                                         42      31     -11
builtin_cd                                            90      74     -16
builtin_source                                        89      72     -17
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(add/remove: 0/0 grow/shrink: 1/3 up/down: 29/-44)            Total: -15 bytes