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.
lukem