pwdx isn't available on OS X

When starting arm and connecting to a tor process that used a relative path for torrc, arm complains it can't find the torrc:

[ARM_WARN] Unable to load torrc (querying tor's pwd failed because the pwdx call failed: 'pwdx' is unavailable)

Instead, on OS X you can use this (quite possibly something more elegant is possible too, but didn't see it in lsof's manpage at first glance):

lsof -a -p $PID -d cwd -Fn | cut -c2- | grep -v $PID