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