Implement .vop_pathconf and .vop_getacl for the .zfs ctldir

zfsctl_common_pathconf will report all the same variables that regular ZFS
volumes report. zfsctl_common_getacl will report an ACL equivalent to 555,
except that you can't read xattrs or edit attributes.

Fixes a bug where "ls .zfs" will occasionally print something like:
ls: .zfs/.: Operation not supported

PR: 225793
Reviewed by: avg
MFC after: 3 weeks
Sponsored by: Spectra Logic Corp
Differential Revision: https://reviews.freebsd.org/D14365