calm/mksetupini changes

[Jon Turney <jon.turney@dronecode.org.uk>]

I've recently deployed some updates to calm, which change a few things 
maintainers may notice:

* test: and prev: lines are no longer permitted in override.hint

Instead of test:, use the test: label in the pvr.hint to mark test 
packages (which 'cygport package-test' or 'all-test' add for you)

prev: doesn't really mean anything anymore (since setup hasn't supported 
specifically installing the prev: version since 2011 (setup version 
2.761. See [1])). keep: is a more flexible replacement.

[1] https://cygwin.com/ml/cygwin-apps/2011-07/msg00064.html

* package retention has been slightly adjusted

By default, we now retain in the relarea up to 3 non-test versions, and 
up to 2 test versions.

This can be tuned on a per-package basis using the various keep*: lines 
in override.hint. See [2]

[2] https://cygwin.com/packaging-hint-files.html#override.hint

* calm now uses a separate pvr-src.hint in the relarea

This should have no externally visible effect (e.g. generated setup.ini 
is not significantly changed), but it enables some complexity (some of 
which isn't implemented properly) to be removed from calm.

At upload, if not present, this hint is created from the corresponding 
pvr.hint by copying (if an install package also exists) or moving (if it 
doesn't) it.

I have written a patch to cygport to generate packages in this form [3], 
but that's not yet in any released cygport.

[3] https://cygwin.com/ml/cygwin-apps/2020-02/msg00050.html

In the unlikely event that:

- you have a private package repository, AND
- you run calm version >= 20200328

You'll need to successfully run 'calm-tool fix-missing-src-hint' on that 
repository before you can use calm or mksetupini.

Additionally, there have been a few other related changes:

* md5.sum files are no longer generated in the relarea.  (use the 
sha512.sum files instead)

* The '!packages' files in maintainer upload directories (which only 
exist for historical reasons, and haven't been updated for a while) have 
been removed. (The authoritative source of this information is [4])

[4] https://cygwin.com/cygwin-pkg-maint