Respect GIT_SSL_NO_VERIFY and http.sslVerify

We recently noticed that libgit2 doesn't respect the `GIT_SSL_NO_VERIFY` environment variable or the `http.sslVerify` config setting. This change adds that support.

At the time the `git_remote` is created, instead of always setting the value of `check_cert` to 1, we now set it to the return value of the helper function `get_check_cert`.