Add struct sr_context to the sr_init() and sr_exit() calls

Add struct sr_context to the sr_init() and sr_exit() calls

libsigrok.git commit b8072700c1bc7d13ba004fd897668b56cec4ac62 adds
struct sr_context to the public API, and changes sr_init() and sr_exit()
to take a struct sr_context ** and struct sr_context * parameter,
respectively.

struct sr_context is opaque, and all sr_init() and sr_exit() calls must
be balanced.

Thanks to the new struct sr_context * the error path code flow can also
be simplifed, and allow a single point in main() to clean up after any
possible program state.