re-enable "-d '?'" as a means to list what loglevels are available (ITS#4666)

doc/man/man8/slapd.8

@@ -91,6 +91,9 @@ Comma-separated arrays of friendly names can be specified to select
 debugging output of the corresponding debugging information.
 All the names recognized by the \fIloglevel\fP directive 
 described in \fBslapd.conf\fP(5) are supported.
+If \fIdebug\-level\fP is \fB?\fP, a list of installed levels is printed,
+and slapd exits.
+
 Remember that if you turn on packet logging, packets containing bind passwords
 will be output, so if you redirect the log to a logfile, that file should
 be read-protected.