files: Receive real fd numbers using recv_fd_from_peer()

Move reopen_fd_as() from receive_fd() to this function.
Note, that recv_fd_from_peer() has other callers, and
all of them are OK with receiving real fds (before,
they received arbitrary fds, and they OK with any fds).

travis-ci: success for Rework file opening scheme to make it asynchronous (rev5)
Signed-off-by: Kirill Tkhai <ktkhai@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>