config: deprecate NoXX-Options

ngircd unfortunately uses several options using double-negation, e.g.

NoIdent = No, NoPam = No, etc.

This renames all options by dropping the "No" prefix, e.g.
"NoIdent = no" becomes "Ident = yes".

The old options will continue to work, but will cause a warning
message.

Also update man pages and default config.

To prevent silly
'Ident = yes' from appearing in  --configtest output in the
'ident support not compiled in and Ident Option not used' case,
make default value depend on feature availability.
If feature is available, enable by default, otherwise disable.

We might consider moving these options to a new
[Feature]

section, or something like that, because none of these options are
essential.

Another possible improvement:

'Ident = yes' option in ngircd.conf causes a warning if ngircd was
built without ident support.

This does not happen with e.g. zeroconf....

doc/sample-ngircd.conf.tmpl

@@ -134,19 +134,18 @@
 	# Allow Pre-Defined Channels only (see Section [Channels])
 	;PredefChannelsOnly = no
 
-	# Don't do any DNS lookups when a client connects to the server.
-	;NoDNS = no
+	# Do any DNS lookups when a client connects to the server.
+	;DNS = yes
 
-	# Don't do any IDENT lookups, even if ngIRCd has been compiled
-	# with support for it.
-	;NoIdent = no
+	# Do any IDENT lookups if ngIRCd has been compiled with support for it.
+	;Ident = yes
 
-	# Don't use PAM, even if ngIRCd has been compiled with support for it.
-	;NoPAM = no
+	# Use PAM if ngIRCd has been compiled with support for it.
+	;PAM = no
 
-	# Don't use ZeroConf service registration, even if ngIRCd has been
+	# Use ZeroConf service registration if ngIRCd has been
 	# compiled with support for it (e.g. Howl, Avahi, Mac OS X).
-	;NoZeroConf = no
+	;ZeroConf = no
 
 	# try to connect to other irc servers using ipv4 and ipv6, if possible
 	;ConnectIPv6 = yes