FAQ: editorial updates

Several language fixes. Several reformats that should make the HTML
generation of this document look better.

Reported-by: Dave Thompson
easy: define away easy_events() for non-debug builds
FTP: remove krb4 support

We've announced this pending removal for a long time and we've
repeatedly asked if anyone would care or if anyone objects. Nobody has
objected. It has probably not even been working for a good while since
nobody has tested/used this code recently.

The stuff in krb4.h that was generic enough to be used by other sources
is now present in security.h