libavformat/tcp.c : add send_buffer_size and recv_buffer_size options

adds two new options that may be set via the dictionary:

- send_buffer_size
- recv_buffer_size

When present, setsockopt() is used with SO_SNDBUF and SO_RCVBUF to set
socket buffer sizes.  I chose to make send and receive independent
because buffering requirements are often asymmetric.

Errors in setting the buffer size mean the socket will use its
default, so they are ignored.

There is no sanity checking on values, as the kernel/socket layers
already impose reasonable limits if asked for something crazy.

Rationale for enlarging receive buffers is to reduce susceptibility
to intermittent network delays/congestion.  I added setting the send
buffer for symmetry.

Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
