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.
@mikemcquaid Thanks! I thought we leaned on whichever the stronger checksum mechanism was that upstream published. Is there a particular reason we lean on sha256 in some places over SHA1 when upstream publishes SHA1, etc? Understand Git leans heavily on SHA1.