fops: allow setting from context creation and introduce lws_select_fops_by_vfs_path

1) There's now a .fops pointer that can be set in the context creation info.  If set, the array of
fops it points to (terminated by an entry with .open = NULL) is walked to find out the best vfs filesystem
path match (comparing the vfs path to fops.path_prefix) for which fops to use.

If none given (.fops is NULL in info) then behaviour is as before, fops are the platform-provided one only.

2) The built in fileserving now walks any array of fops looking for the best fops match automatically.

3) lws_plat_file_... apis are renamed to lws_vfs_file_...