pkgfs_open: follow symlinks

pkgfs_open: follow symlinks

Caller is not interested in symlinks follow them.
Throw an error if too many links encountered.

Reviewed by:	stevek
Sponsored by:	Juniper Networks
--This line, and those below, will be ignored--
> Description of fields to fill in above:                     76 columns --|
> PR:                       If a GNATS PR is affected by the change.
> Differential Revision:    https://reviews.freebsd.org/D### (*full* phabric URL needed).
> Submitted by:             If someone else sent in the change.
> Reviewed by:              If someone else reviewed your modification.
> Approved by:              If you needed approval for this commit.
> Obtained from:            If the change is from a third party.
> MFC after:                N [day[s]|week[s]|month[s]].  Request a reminder email.
> MFH:                      Ports tree branch name.  Request approval for merge.
> Relnotes:                 Set to 'yes' for mention in release notes.
> Security:                 Vulnerability reference (one per line) or description.
> Sponsored by:             If the change was sponsored by an organization.
> Empty fields above will be automatically removed.

Change-Id: I4ef92ff256f503c620dc5bba79ed93b32cb2330d