When the stream writing function was written, it assume that
libssh2_channel_write() would always write all of the data to the
wire. This is only true for the first 32k of data, which it tries to
fit into one ssh packet.

Since it can perform short writes, call it in a loop like we do for
send(), advancing the buffer offset.

Don't use read-write accounts for the tests

netops: init OpenSSL once under lock

The assembla failure we were seeing referred to a private repository,
which is not what is there at the moment.

This reverts 1fd21b0342f

Don't write in plaintext the password of an account which has full
control over the repository. Instead use an account with read-only
access.

We want to check whether the credentials callback is NULL, not whether
the payload is.

Bring together all of the OpenSSL initialization to
git_threads_init() so it's together and doesn't need locks.

Moving it here also gives us libssh2 thread safety (when built against
openssl).

OpenSSL's tests init everything in the main thread, so let's do that.