cmd/syncthing, lib/config: Enable HTTP CPU/heap profile collection for users

This adds a config to enable debug functions on the API server, which is
by default disabled. When enabled, the /rest/debug things become
available and become available without requiring a CSRF token (although
authentication is required if configured).

We also add a new endpoint /rest/debug/cpuprof?duration=15s (with the
duration being configurable, defaulting to 30s). This runs a CPU profile
for the duration and returns it as a file. It sets headers so that a
browser will save the file with an informative name.

The same is done for heap profiles, /rest/debug/heapprof, which does not
take any parameters.

The purpose of this is that any user can enable debugging under
advanced, then point their browser to the endpoint above and get a file
that contains a CPU or heap profile we can use, with the filename
telling us what version and architecture the profile is from.

On the command line, this becomes

    curl -O -J http://localhost:8082/rest/debug/cpuprof?duration=5s
    curl: Saved to filename
    'syncthing-cpu-darwin-amd64-v0.14.3+4-g935bcc0-110307.pprof'

GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/3467

lib/config/guiconfiguration.go

@@ -21,6 +21,7 @@ type GUIConfiguration struct {
 	APIKey              string `xml:"apikey,omitempty" json:"apiKey"`
 	InsecureAdminAccess bool   `xml:"insecureAdminAccess,omitempty" json:"insecureAdminAccess"`
 	Theme               string `xml:"theme" json:"theme" default:"default"`
+	Debugging           bool   `xml:"debugging,attr" json:"debugging"`
 }
 
 func (c GUIConfiguration) Address() string {