Merge branch 'warnings' into blead

These several commits silence warnings specific to Microsoft compilers.
They are bogus warnings as far as we can determine, but persist through
many versions of the compilers.

commit 6879a07bc8b8e75c35d0e3b765ea718cc1c7aeb2
*Author: Tomasz Konojacki <me@xenu.pl>
 Date:   Thu Apr 4 22:39:38 2019 +0200

     implement diagnostics ignore/restore macros for Visual C++

gave the easy infrastructure to turn off such warnings, but adding lines
to take advantage of this makes the code harder to read, and we came up
with alternative methods to shut the compiler up that are less
intrusive, involving casting, and subtracting from 0 instead of a unary
minus, that should be valid across platforms.

There were several other warnings I left in, because it would have
required more research than I have time for right now to be sure that
they are bogus.