darwinssl: reform OS-specific #defines

This doesn't need to be in the release notes. I cleaned up a lot of the #if
lines in the code to use MAC_OS_X_VERSION_MIN_REQUIRED and
MAC_OS_X_VERSION_MAX_ALLOWED instead of checking for whether things like
__MAC_10_6 or whatever were defined, because for some SDKs Apple has released
they were defined out of place.