gecko-dev/tools/update-programs

This directory defines a build project for focused work on the "update
programs": programs owned or maintained by the Install/Update team
that are standalone binaries (i.e., not part of the Firefox binary
proper).

To use this build project, prepare a minimal mozconfig with
```
ac_add_options --enable-project=tools/update-programs
```

Depending on the mozconfig options and host and target OS, some of the
following will be built:

1. the maintenance service (when `--enable-maintenance-service`);
2. the updater binary (when `MOZ_UPDATER=1`);
3. the Windows Default Browser Agent (when `--enable-default-browser-agent`);
4. the Background Update Agent (when `--enable-update-agent`).

Packaging the installer and uninstaller is not yet supported: instead,
use an (artifact) build with `--enable-application=browser`.