Mako88 reports that using `-pilot Mixed` on Windows causes crashes
during multiplayer setup, but `-pilot mixed` does not crash. This is
not reproducible on case sensitive filesystems, probably because case
sensitive systems recognize that `Mixed.plr` and `mixed.plr` are not the
same file. In both cases, the underlying plr file name was all
lowercase. Coerce the pilot name to lowercase when it is converted to a
plr file name. This also fixes a minor bug where the check for a
user-specified extension of `.plr` would recognize `-pilot alice.plr`,
but not recognize `-pilot alice.PLR`.
This is a workaround for a bug in the network code, which becomes
confused and crashes when the player's callsign contains mixed case.
Reported-by: Mako88 <https://github.com/dxx-rebirth/dxx-rebirth/issues/188>
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