resource: New internal API for accessing resource files

resource: New internal API for accessing resource files

The resource API provides a generic means for accessing resources
that are bundled with sigrok, such as device firmware files. Since
the manner of resource bundling is platform-dependent, users of
libsigrok may override the functions used to open, close and read
a resource. The default implementation accesses resources as files
located in one of the XDG data directories or a directory defined
at compile time.