mosh: use brewed openssl as default

The openssl library bundled with OS X operation system is deprecated. It is better to link against the brewed openssl library instead of the system one.

In addition the option to link against the system one is offered.
@DomT4 For future reference that's not actually the case; I don't prefer `sha256` over `sha1` unless it's easier to use (i.e. displayed on the download page and `sha1` isn't. Git's security and thus Homebrew's update security relies on SHA-1 anyway.