lei: prefer IO::FDPass over our Inline::C recv_3fds
While our recv_3fds() implementation is more efficient
syscall-wise, loading Inline takes nearly 50ms on my machine
even after Inline::C memoizes the build. The current ~20ms in
the fast path is barely acceptable to me, and 50ms would be
unusable.
Eventually, script/lei may invoke tcc(1) or cc(1) directly in
the fast path, but it needs @INC for the slow path, at least.
We'll encode the number of FDs into the socket name allow
parallel installations, for now.