Support setting capabilities in ServePrincipal()

This is done properly in the carddav and caldav packages, but the custom
function does not know what the user intends to serve, so it must be
passed in from the user. Without this, certain clients (e.g. DAVx5)
will be unable to discover endpoints served this way.

Also slightly extend the supported methods returned on OPTIONS requests.
REPORT is properly supported, the others are mostly for not giving
clients the impression that the resources are read-only.

caldav/caldav.go

@@ -12,6 +12,8 @@ import (
 	"github.com/emersion/go-webdav/internal"
 )
 
+var CapabilityCalendar = webdav.Capability("calendar-access")
+
 func NewCalendarHomeSet(path string) webdav.BackendSuppliedHomeSet {
 	return &calendarHomeSet{Href: internal.Href{Path: path}}
 }