RecentChange: Use constants for the $noudp parameter of save()

RecentChange: Use constants for the $noudp parameter of save()

It is a boolean parameter that is confusing for three reasons:

* It's a boolean parameter, given parameters are unnamed in PHP,
  these are always poor UX for call sites.

* It's negated ("noudp"). save(true) means no feeds events,
  save(false) [default] means sending events to feeds.

* To overcome this problem, typical use was to pass a free-form
  string that self-documents the intended behaviour,
  e.g. `save('pleasenoudp')`, any string casts to true.

Fix this by moving the booleans to constants and use those
instead. For compatiblity, keep the negation internally,
although it's hidden from regular usage. Also document
the string hack, deprecate it, and update callers.

Change-Id: Ia57c86b38bf50cb4ec580f42a6b1ca798fcf781a