files: Kill fd_open_state::receive_fd stage

Since "receive" stage is used only for slave fds
and nobody depends on slave fds receiving is finished,
we may move it functionality in "post_open" stage.
This just makes slave fds to be received a little bit later.

In other words, only masters have post_open stage,
and only slaves have receive stage. So, in the case of
A and B files:

A->open
B->open
A->recv
A->post_open
B->recv
B->post_open

A->post_open can't depend on B->recv. This follows
just from analyzing of all file types post_open methods.

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>