Add new systemd user unit services

This commit for the new "systemd" (1) snowblock includes user service (2)
units (3) to

1. automount the "sync" gocryptfs (4) volume located in the Dropbox (5)
folder receiving the password stored in the GNOME Keyring (6) via
the "secret-tool" ("libsecret" (7)).
2. automatically run a instance of the "ssh-agent" (8) using the
"$SSH_AUTH_SOCK" and let systemd manage the "$SSH_AGENT_PID env.

>>> snowsaw core plugin configuration

>>>> clean

The target path "~/.config/systemd/user" is cleaned before the "link"
core plugin execution.

>>>> link

The target paths "~/.config/systemd/user" for all unit files will be
created by snowsaw if they doesn't exist yet.

The "igloosync-dropbox.service" unit file will be linked for the
"archbook" and "igloo" hosts while the "ssh-agent.service" file is
linked by default.

References:
  (1) https://wiki.archlinux.org/index.php/Systemd
  (2) https://www.freedesktop.org/software/systemd/man/systemd.service.html
  (3) https://www.freedesktop.org/software/systemd/man/systemd.unit.html
  (4) https://github.com/rfjakob/gocryptfs
  (5) https://www.dropbox.com
  (6) https://wiki.archlinux.org/index.php/GNOME/Keyring
  (7) https://wiki.gnome.org/Projects/Libsecret
  (8) https://wiki.archlinux.org/index.php/SSH_keys#ssh-agent

GH-58

snowblocks/systemd/README.md

@@ -0,0 +1,32 @@
+* [Units](#units)
+  * [Services](#services)
+    * [igloosync-dropbox](#igloosync-dropbox)
+
+# Units
+
+## Services
+
+### igloosync-dropbox
+
+> The [gocryptfs][gocryptfs-gh] password **must be stored in the** [GNOME Keyring][archwiki-keyring] **before enabling this user service** in order to receive the value from the [`secret-tool`][gnome-libsecret]!
+
+The stored password must be assigned with the following attribute-value pair:
+
+* attribute: `igloosync`
+* value: `dropbox`
+
+To store the password run
+
+```sh
+secret-tool store --label="igloosync-dropbox" igloosync dropbox
+```
+
+The password can be received from the `secret-tool` by running
+
+```sh
+secret-tool lookup igloosync dropbox
+```
+
+[archwiki-keyring]: https://wiki.archlinux.org/index.php/GNOME/Keyring
+[gnome-libsecret]: https://wiki.gnome.org/Projects/Libsecret
+[gocryptfs-gh]: https://github.com/rfjakob/gocryptfs

snowblocks/systemd/snowblock.json

@@ -0,0 +1,20 @@
+[
+  {
+    "clean": ["~/.config/systemd/user"]
+  },
+  {
+    "link": {
+      "~/.config/systemd/user/igloosync-dropbox.service": {
+        "create": true,
+        "hosts": {
+          "archbook": "units/user/igloosync-dropbox.service",
+          "igloo": "units/user/igloosync-dropbox.service"
+        }
+      },
+      "~/.config/systemd/user/ssh-agent.service": {
+        "create": true,
+        "path": "units/user/ssh-agent.service"
+      }
+    }
+  }
+]

snowblocks/systemd/units/user/igloosync-dropbox.service

@@ -0,0 +1,12 @@
+[Unit]
+Description=Dropbox Synchronization (igloosync-dropbox)
+After=network.target local-fs.target
+
+[Service]
+Type=forking
+ExecStart=/usr/bin/gocryptfs -allow_other -extpass="secret-tool lookup igloosync dropbox" %h/cloud/.dropbox/Dropbox/sync %h/cloud/dropbox
+Restart=on-failure
+RestartSec=5
+
+[Install]
+WantedBy=default.target

snowblocks/systemd/units/user/ssh-agent.service

@@ -0,0 +1,10 @@
+[Unit]
+Description=SSH Key Agent
+
+[Service]
+Type=forking
+Environment=SSH_AUTH_SOCK=%t/ssh-agent.socket
+ExecStart=/usr/bin/ssh-agent -a $SSH_AUTH_SOCK
+
+[Install]
+WantedBy=default.target