bin/mdw-build: Abolish (direct) use of `mdw-conf'.

bin/mdw-build: Abolish (direct) use of `mdw-conf'.

Values are now configured from the command line or the configuration
file.  Since the configuration file is a shell fragment, it can invoke
`mdw-conf' itself for compatibility, and, indeed, I now have such a
configuration file.  The default `mdw.conf' file now has the necessary
extra keys in it.

This involves a little new machinery in `mdw-build' itself.  Some new
flags are introduced, to control whether the outputs should be
signed (and if so, which key should be used); and whether a Debian
upload should be performed (and if so, which target to use).
Correspondingly, the existing `upload' option has grown a configurable
remote path to control where the files should be written.