Truly support custom PREFIX= install

* Before, doing `$ PREFIX=/usr/local make install` would install files to`/usr/local/usr/local..` which is wrong
* With this PR, files will be installed to the expected location e.g. `/usr/local/etc/restic`
* `Makefile` almost completely rewritten
   * As e.g. `default.env` would source `_global.env`, `default.env` must be edited to find the right location of `_global.env` depending on what `$PREFIX` was set to.
   * see documented build stages in the `Makefile` itself.
   *  Made sure that the rules are correct so that only modifed files are installed, not all at once unnecessarily like before.
* A sub-goal was that the [PKGBUILD](https://aur.archlinux.org/cgit/aur.git/tree/PKGBUILD?h=restic-systemd-automatic-backup#n20) for Arch should not need to do any custom install configuration, to keep everything easier to maintain. `$ make install` should work out of the box for Arch.
* Additionally added the `-b` flag to `install(1)` that makes a backup of existing `etc/restic/*` files before installing a newer version.

Fixes #49

usr/lib/systemd/system/nm-unmetered-connection.service

@@ -0,0 +1,6 @@
+[Unit]
+Description=Check if the current NetworkManager connection is metered
+
+[Service]
+Type=oneshot
+ExecStart=$INSTALL_PREFIX/sbin/nm-unmetered-connection.sh

usr/lib/systemd/system/restic-backup@.service

@@ -0,0 +1,13 @@
+[Unit]
+Description=Backup with restic to Backblaze B2
+OnFailure=status-email-user@%n.service
+Requires=nm-unmetered-connection.service
+
+[Service]
+Type=simple
+Nice=10
+# $HOME or $XDG_CACHE_HOME must be set for restic to find /root/.cache/restic/
+Environment="HOME=/root"
+# The random sleep (in seconds) is in the case of multiple backup profiles. Many restic instances started at the same time could case high load or network bandwith usage.
+# `systemd-cat` allows showing the restic output to the systemd journal
+ExecStart=bash -c 'sleep $(shuf -i 0-300 -n 1) && source $INSTALL_PREFIX/etc/restic/%I.env && $INSTALL_PREFIX/sbin/restic_backup.sh | systemd-cat'

usr/lib/systemd/system/restic-backup@.timer

@@ -0,0 +1,10 @@
+[Unit]
+Description=Backup with restic on schedule
+Requires=nm-unmetered-connection.service
+
+[Timer]
+OnCalendar=daily
+Persistent=true
+
+[Install]
+WantedBy=timers.target

usr/lib/systemd/system/restic-check@.service

@@ -0,0 +1,11 @@
+[Unit]
+Description=Check restic backup Backblaze B2 for errors
+OnFailure=status-email-user@%n.service
+Conflicts=restic-backup.service
+Requires=nm-unmetered-connection.service
+
+[Service]
+Type=simple
+Nice=10
+# `systemd-cat` allows showing the restic output to the systemd journal
+ExecStart=bash -c 'source $INSTALL_PREFIX/etc/restic/%I.env && $INSTALL_PREFIX/sbin/restic_check.sh | systemd-cat'

usr/lib/systemd/system/restic-check@.timer

@@ -0,0 +1,10 @@
+[Unit]
+Description=Check restic backup Backblaze B2 for errors on a schedule
+Requires=nm-unmetered-connection.service
+
+[Timer]
+OnCalendar=monthly
+Persistent=true
+
+[Install]
+WantedBy=timers.target

usr/lib/systemd/system/status-email-user@.service

@@ -0,0 +1,11 @@
+# Source: https://serverfault.com/questions/876233/how-to-send-an-email-if-a-systemd-service-is-restarted
+# Source: https://wiki.archlinux.org/index.php/Systemd/Timers#MAILTO
+
+[Unit]
+Description=Send status email for %i to user
+
+[Service]
+Type=oneshot
+ExecStart=$INSTALL_PREFIX/sbin/systemd-email abc@gmail.com %i
+User=root
+Group=systemd-journal