lib/config, lib/connections: Add optional connection limits (fixes #7176) (#7223)

This adds two new configuration options:

    // The number of connections at which we stop trying to connect to more
    // devices, zero meaning no limit. Does not affect incoming connections.
    ConnectionLimitEnough int

    // The maximum number of connections which we will allow in total, zero
    // meaning no limit. Affects incoming connections and prevents
    // attempting outgoing connections.
    ConnectionLimitMax int

These can be used to limit the number of concurrent connections in
various ways.

lib/config/optionsconfiguration.go

@@ -47,6 +47,14 @@ func (opts *OptionsConfiguration) prepare(guiPWIsSet bool) {
 			}
 		}
 	}
+
+	// Negative limits are meaningless, zero means unlimited.
+	if opts.ConnectionLimitEnough < 0 {
+		opts.ConnectionLimitEnough = 0
+	}
+	if opts.ConnectionLimitMax < 0 {
+		opts.ConnectionLimitMax = 0
+	}
 }
 
 // RequiresRestartOnly returns a copy with only the attributes that require
@@ -178,3 +186,17 @@ func (opts OptionsConfiguration) FeatureFlag(name string) bool {
 
 	return false
 }
+
+// LowestConnectionLimit is the lower of ConnectionLimitEnough or
+// ConnectionLimitMax, or whichever of them is actually set if only one of
+// them is set. It's the point where we should stop dialling.
+func (opts OptionsConfiguration) LowestConnectionLimit() int {
+	limit := opts.ConnectionLimitEnough
+	if limit == 0 || (opts.ConnectionLimitMax != 0 && opts.ConnectionLimitMax < limit) {
+		// It doesn't really make sense to set Max lower than Enough but
+		// someone might do it while experimenting and it's easy for us to
+		// do the right thing.
+		limit = opts.ConnectionLimitMax
+	}
+	return limit
+}