man: Remove client attribute description

Remove attribute description from clients.conf(5) source as it is
outdated, lists just a few attributes and wasn't updated for more than a
year. Refer to clients.conf file itself, instead.
man: Remove references to naslist and clients

Remove references to non-existent naslist and clients manpages as they
are confusing.
Use correct type for log_lvl and add prefix to rdebug_pair
Make IP resolution errors a bit clearer
Add vp_length macro (makes it easier to backport changes from master)
Change references to vp->length to vp->vp_length
Delete state correctly
rlm_cache_rbtree isn't in "stable".  It's ALWAYS built.

It has no configure script, so we don't need to recurse
into it's subdirectory to configure it
Note recent changes
Merge pull request #847 from spbnick/man_clients_update

clients(5) update
Escape backslashes only when necessary
Fix issue that caused radclient to default to sending Accounting-Request when auto was specified and Packet-Type was set
Slightly better debug output
Duplicate init
Formatting
Update range of cached TLS attributes
Cache Stripped-User-Domain too
Note on why we don't need to escaping for execve
Formatting
Escape filename expansions

Attributes used in xlat expansions involving filenames will now be subject to tight escaping rules to prevent injection attacks, and to allow maximum compatibility.

A little scary this wasn't done already, but for most systems it would have been pretty hard to exploit in a useful way.