snap: Start snap support.

- Create a snapcraft.yaml file in snap directory.
  Couldn't put this in packaging directory.
- Use make-snapcraft.py to generate the snapcraft file.
- Use make-snap to make the snap from the snapcraft file.
- Eris is confined to only access projects in home.
- os.setpriority doesn't work in snaps.
- eris-webserver is run as "eris.webserver".

packaging/make-snapcraft.py

@@ -0,0 +1,70 @@
+#!/usr/bin/env python3.9
+# -*- coding: utf-8 -*-
+
+
+import textwrap
+
+import eris.__main__
+import eris.tools as tools
+
+
+def main():
+    lines = eris.__main__.__doc__.splitlines()
+    summary = lines[2]
+    description = textwrap.indent("\n".join(lines[2:]), "  ")
+    print(f"""\
+name: eris
+base: core20
+version: 2021.11.29
+summary: {summary}
+description: |
+{description}
+
+grade: devel
+confinement: strict
+
+apps:
+  eris:
+    command: bin/eris
+    environment:
+      PYTHONPATH: $SNAP/lib/python3.9/site-packages:$SNAP/lib/python3.9/dist-packages:$PYTHONPATH
+    plugs:
+      - home
+  webserver:
+    command: bin/eris-webserver
+    environment:
+      PYTHONPATH: $SNAP/lib/python3.9/site-packages:$SNAP/lib/python3.9/dist-packages:$PYTHONPATH
+    plugs:
+      - home
+      - network-bind
+
+passthrough:
+  layout:
+    $SNAP/usr/bin/php:
+      symlink: $SNAP/usr/bin/php7.4
+
+parts:
+  eris:
+    plugin: python
+    source: .
+    python-packages:
+      - ./termstr
+      - ./lscolors
+      - ./fill3
+      - ./eris
+    requirements:
+      - ./termstr/requirements.txt
+      - ./fill3/requirements.txt
+      - ./eris/requirements.txt
+    build-environment:
+      - SNAPCRAFT_PYTHON_INTERPRETER: python3.9
+    stage-packages:
+      - python3.9-venv
+      - python3-distutils
+      - python3-pkg-resources
+      - """, end="")
+    print("\n      - ".join(sorted(tools.dependencies())))
+
+
+if __name__ == "__main__":
+    main()