Introduced in
6dcda3595e
(Add recovery for randomly closed socket desciptors.)
By copying CurlSocketInfo* into mCopiedFileDescriptors, it was possible
that we accessed a deleted CurlSocketInfo for it's filedescriptor,
returning a random value, which, when passed to FD_ISSET could cause a
SIGSEGV.
I reverted this change and now store the literal socket descriptors in
mCopiedFileDescriptors again. It is not a problem when libcurl deletes
the corresponding CurlSocketInfo (through a callback to
MultiHandle::socket_callback) in that case, because the fd_set we test
against isn't updated as a result of that, not until we're down with
mCopiedFileDescriptors (at the re-entry of select()).