block/curl: check error return of curl_global_init()

If curl_global_init() fails, per the documentation no other curl
functions may be called, so make sure to check the return value.

Also, some minor changes to the initialization latch variable 'inited':

- Make it static in the file, for clarity
- Change the name for clarity
- Make it a bool

Signed-off-by: Jeff Cody <jcody@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Richard W.M. Jones <rjones@redhat.com>
Reviewed-by: Darren Kenny <darren.kenny@oracle.com>
Signed-off-by: Jeff Cody <jcody@redhat.com>
