replace LWS_MAX_SOCKET_IO_BUF with context creation info pt_serv_buf_size

replace LWS_MAX_SOCKET_IO_BUF with context creation info pt_serv_buf_size

This makes it easy for user code to choose the size of the per-thread
buffer used by various things in lws, including file transfer chunking.

Previously it was 4096, if you leave info.pt_serv_buf_size as zero that
is still the default.

With some caveats, you can increase transfer efficiency by increasing it
to, eg, 128KiB, if that makes sense for your memory situation.

Signed-off-by: Andy Green <andy@warmcat.com>