Copy stable/9 to releng/9.3 as part of the 9.3-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

MFC r240252, r241541, r241543, r245756:

r240252: (eadler)
- Remove documentation and www cvsup files as they are no longer
useful with the switch to subversion.

r241541: (joel)
- Minor mdoc improvements. Also remove unnecessary csup reference.

r241543: (eadler)
- Bump .Dd

r245756:
- Mark SUP_UPDATE as deprecated in make.conf(5), providing
instructions to use SVN_UPDATE or freebsd-update(8).
- While here, remove bogus NO_WWWUPDATE.

MFC r231244:
- Fix some Xr references:

- - ada(4): ad(4) - removed, ada(4) would be a self-referencing entry
- - cd(4): ad(4) -> ada(4)
- - da(4): ad(4) -> ada(4)
- - DEVICE_PROBE(9): ugen(5) -> ugen(4)
- - ed(4): dhclinet(8) -> dhclient(8) (typo)
- - lmc(4): Netgraph(4) -> netgraph(4)
- - security(7): rc.conf(8) -> rc.conf(5)
- - sfxge(4): cpuset(8) -> cpuset(1)
- - sbp(4): sysctl(1) -> sysctl(8)
- - portindex(5): build(1) -> build(7)
- - u3g(4): usbconfig(5) -> usbconfig(8)
- - usb_quirk(4): usbconfig(5) -> usbconfig(8)

Copy head to stable/9 as part of 9.0-RELEASE release cycle.

Approved by: re (implicit)

- Re-order SEE ALSO entries

Noticed by: brueffer
Approved by: trhodes

- Add initial version of portindex(5) manual page

PR: docs/70652
Submitted by: Paul Armstrong <psa otoh.org>
Reviewed by: pav keramida trhodes
Approved by: keramida
MFC after: 3 days