lei_xsearch: transfer 4 FDs internally, drop IO::FDPass
It's easier to make the code more generic by transferring
all four FDs (std(in|out|err) + socket) instead of omitting
stdin.
We'll be reading from stdin on some imports, and possibly
outputting to stdout, so omitting stdin now would needlessly
complicate things.
The differences with IO::FDPass "1" code paths and the "4"
code paths used by Inline::C and Socket::MsgHdr are far too
much to support and test at the moment.