remove need for filepath buffer on http file serve

This gets rid of the stack buffer while serving files, and the
PATH_MAX char array that used to hold the filepath in the wsi.

It holds an extra file descriptor open while serving the file,
however it attempts to stuff the socket with as much of the
file as it can take.  For files of a few KB, that typically
completes (without blocking) in the call to
libwebsockets_serve_http_file() and then closes the file
descriptor before returning.

Signed-off-by: Andy Green <andy.green@linaro.org>
use context service buf in place of large stack arrays

Signed-off-by: Andy Green <andy.green@linaro.org>
remove all PATH_MAX or MAX_PATH

Signed-off-by: Andy Green <andy.green@linaro.org>
account for context in static allocation figure

Signed-off-by: Andy Green <andy.green@linaro.org>
remove fixed rx buffer allow definition per protocol

A new protocol member is defined that controls the size of rx
buffer allocation per connection.  For compatibility 0 size
allocates 4096, but you should adapt your protocol definition
array in the user code to declare an appropriate value.

See the changelog for more detail.

The advantage is the rx frame buffer size is now tailored to
what is expected from the protocol, rather than being fixed
to a default of 4096.  If your protocol only sends frames of
a dozen bytes this allows you to only allocate an rx frame
buffer of the same size.

For example the per-connection allocation (excluding headers)
for the test server fell from ~4500 to < 750 bytes with this.

Signed-off-by: Andy Green <andy.green@linaro.org>