Correct version string on Mac OS X
Fixed the output of P4.identify for P4Ruby built with Ruby 1.9.2 on
Mac OS X, so that it now reads 'P4RUBY/DARWIN10X86_64/...'.
Previously, even if $p4osplat was set, it could get overwritten with the
output from 'uname -p', which erroneously set it to 'X86'. Now, we check
if '$p4osplat' is set, and only if it isn't do we fall back on 'uname -p'.
Otherwise, we assume that CONFIG[ 'arch' ] is correct.