xt_quota2: support packet counting; add manpage

extensions/libxt_quota2.man

@@ -0,0 +1,31 @@
+The "quota2" implements a named counter which can be increased or decreased
+on a per-match basis. Available modes are packet counting or byte counting.
+The value of the counter can be read and reset through procfs, thereby making
+this match a minimalist accounting tool.
+.PP
+When counting down from the initial quota, the counter will stop at 0 and
+the match will return false, just like the original "quota" match. In growing
+(upcounting) mode, it will always return true.
+.TP
+\fB--grow\fP
+Count upwards instead of downwards.
+.TP
+\fB--name\fP \fIname\fP
+Assign the counter a specific name. This option must be present, as an empty
+name is not allowed. Names starting with a dot or names containing a slash are
+prohibited.
+.TP
+[\fB!\fP] \fB--quota\fP \fIiq\fP
+Specify the initial quota for this counter. If the counter already exists,
+it is not reset. An "!" may be used to invert the result of the match. The
+negation has no effect when \fB--grow\fP is used.
+.TP
+\fB--packets\fP
+Count packets instead of bytes that passed the quota2 match.
+.PP
+Because counters in quota2 can be shared, you can combine them for various
+purposes, for example, a bytebucket filter that only lets as much traffic go
+out as has come in:
+.PP
+-A INPUT -p tcp --dport 6881 -m quota --name bt --grow
+-A OUTPUT -p tcp --sport 6881 -m quota --name bt