files: Rework the way fdinfos are collected

Now fdinfos are collected independently from reg files and
sockets. During tihs collect we effectively create the mirroring
list of both by checking which type-IDs are added first.

Fix this by removing the fdinfo_desc and attaching fds directly
to collected reg files and sockets. Pipes and unix sockets will
be reworked in the same manner soon.

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>