deprecate backend method names for elliptic curve number loading

fixes #1270
skip deprecated tests on platforms that don't support ec (old rhel)
multibackend deprecated method needs to call the backend deprecated method
fix test coverage on multibackend deprecated methods
return from deprecated method
remove duplicate _skip_curve_unsupported
EC public/private key loader functions try deprecated as fallback
Merge pull request #1348 from reaperhulk/improve-naming-consistency

deprecate backend method names for elliptic curve number loading