Extcap: Create our fifo in a temporary directory.

Instead of creating a temp file, unlinking it, and creating a fifo with
the same name, add create_tempdir() so that we can create a temporary
directory and create a fifo inside that.

This should avoid a race condition in Carbon Black Cloud antivirus,
which if the timing is right, will stat the initial temporary *file*,
miss the fact that it's been replaced with a *fifo*, and open and steal
data^W^W read from it, leaving dumpcap to contend with the truncated
remains.

Adding the unexpected magic number to cap_pipe_open_live()'s error
message helped to debug this. Leave it in since it's handy to have in
that case.

Ping #15587

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