When we're assessing whether a bottle is relocatable, we shouldn't have
to descend into symlink paths we encounter. This is supposed to be the
default behavior but it doesn't appear to be (perhaps because we pass a
symlink to the keg on the command line?).

All of the switches that control this behavior differ between BSD and
GNU grep, so sniff the grep flavor first.

ln_sf does the right thing when `dest` is a symlink pointing to a file:
the symlink gets overwritten with a link pointing to the new src. But
when dest points to a directory, we create a new symlink inside the
folder dest points to, which doesn't help us at all.