b4: Rework handling of output file naming

When passing a name like "a.mbx" to b4-am's --mbox-name, only the "a"
is retained; b4-am unconditionally appends the ".mbx".  Avoid passing
a name with ".mbx" to make it clearer that the caller does not control
the extension.

Reduce the base name to "m" to avoid a message ID ending with ".*"
being treated as an extension, which would require analyzing the
message ID in order to guess what output name b4-am will use.

Finally, name the custom thread mbox with "-piem" rather than
"-thread" (which was chosen to mimic b4-mbox) so it's more obvious who
created the file when later inspecting the directory.

0 files changed, 0 insertions, 0 deletions