lws_plat_fd implement platform default handlers

This is a rewrite of the patch from Soapyman here

https://github.com/warmcat/libwebsockets/pull/363



The main changes compared to Soapyman's original patch are

 - There's no new stuff in the info struct user code does any overrides
   it may want to do explicitly after lws_context_create returns

 - User overrides for file ops can call through (subclass) to the original
   platform implementation using lws_get_fops_plat()

 - A typedef is provided for plat-specific fd type

 - Public helpers are provided to allow user code to be platform-independent
   about file access, using the lws platform file operations underneath:

static inline lws_filefd_type
lws_plat_file_open(struct lws_plat_file_ops *fops, const char *filename,
		   unsigned long *filelen, int flags)

static inline int
lws_plat_file_close(struct lws_plat_file_ops *fops, lws_filefd_type fd)

static inline unsigned long
lws_plat_file_seek_cur(struct lws_plat_file_ops *fops, lws_filefd_type fd,
		       long offset_from_cur_pos)

static inline int
lws_plat_file_read(struct lws_plat_file_ops *fops, lws_filefd_type fd,
		   unsigned long *amount, unsigned char *buf, unsigned long len)

static inline int
lws_plat_file_write(struct lws_plat_file_ops *fops, lws_filefd_type fd,
		    unsigned long *amount, unsigned char *buf, unsigned long len)

There's example documentation and implementation in the test server.

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