MFC r235982:

MFC r235982:
Add tunable/sysctl kern.cam.pmp.hide_special, controlling whether special
PMP ports such as PMP configuration or SEMB should be exposed or hidden.
These ports were always hidden before as useless and sometimes promatic.
But with updated ses driver supporting SEMB it is no longer so straight.
Keep ports hidden by default to avoid probe request ttimeouts if SEP is
not connected to PMP's SEMB via I2C, that is very often situation.

git-svn-id: svn://svn.freebsd.org/base/stable/8@236766 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f