Create overloads for API init functions to specify custom hosts.

[Jamiras]

Deprecates rc_api_set_host and rc_api_set_image_host. Custom hosts are now passed to _hosted alternatives for each API _init function.

Passing NULL to the _hosted functions will use the default hosts. The non-_hosted versions pass the custom hosts assigned by calling rc_api_set_host and rc_api_set_image_host. Once those functions are fully removed, the non-_hosted versions will just pass NULL and the default hosts will be used.

The existing rc_client_set_host function can be used to specify a custom host for use by a single rc_client instance (it was previously a wrapper for calling the above deprecated functions). It shouldn't be necessary to specify an image host, as #393 and #395 expose full URLs to images that should be used instead of trying to build a URL.