Modified the rules for vendor ids:

• explictly say not to edit infrastructure for vendor ids (not just the ids).
• say to enclose vendor ids and their infrastucture in ``#if 0'', and partly explain why.
• don't set a bad example by mangling the Berkeley id infrastructure from `static char sccsid[] ...'' to `__RCSID(...)''.
• show a blank line between the vendor id cruft and the FreeeBSD if cruft in the example.
• relaxed the rule about adding "From: " to say that "From: " is actually useful if the file has been renamed.
• minor English improvements.

Discussed with: obrien