solution with less code replication. use realpath() in logcmd() so
that all logged filenames are sane.
* support `REST STREAM' in `FEAT' reply (from draft-ietf-ftpext-mlst-05)
* in 'HELP', suffix unimplemented commands with `-' instead of `*'; the
former is easier to differentiate from `+'.
* deprecate curdir() now that logcmd() doesn't use it.
* ensure all filename buffers are at least MAXPATHLEN+1 in size.
* move jmp_buf errcatch out of extern.h, removing need to #include <setjmp.h>
in every file.
problem; move `hasyyerrored' state flag out of yylex() so that
check_{login,modify} can also set it.
* check result of check_login for PORT command
* set initial timeout before the "setjmp(); for(;;) yyparse()",
otherwise an invalid command after login incorrectly sets the timeout
to 5 minutes (rather than what was set in ftpd.conf)
* replace (char *)0 with NULL
* move yyerror() from ftpd.c to ftpcmd.y
* remove need for -Dunix, by using the version string from ftpd.c
(instead of `BSD-199506')
* move all extern-ed vars into extern.h
