Bug 1754040 - Clean up common shared WriteSegments helper functions, r=necko-reviewers,xpcom-reviewers,mccr8

These helper methods are similar to the related ones for
nsIInputStream::ReadSegments, and can be used to implement
nsIOutputStream::Write and nsIOutputStream::WriteFrom in terms of
WriteSegments.

There were various places which used manual copies of these methods, which are
being unified.

Differential Revision: https://phabricator.services.mozilla.com/D138334