Fixes#80.
Disable by default the symlinks resolution, making a symlink and its
target 2 different entries in the database. Adds the
_ZO_RESOLVE_SYMLINKS env variable to re-enable it.
Example:
/tmp/foo-target is a directory
/tmp/foo symlinks to /tmp/foo-target
With _ZO_RESOLVE_SYMLINKS=1, `z add /tmp/foo` adds `/tmp/foo-target` in the database.
With _ZO_RESOLVE_SYMLINKS=0 or unset, `z add /tmp/foo` adds `/tmp/foo` in the database.