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

sbin/restic_check.sh

@@ -0,0 +1,42 @@
+#!/usr/bin/env bash
+# Check the backups made with restic to Backblaze B2 for errors.
+# See restic_backup.sh on how this script is run (as it's analogous for this script).
+
+# Exit on error, unset var, pipe failure
+set -euo pipefail
+
+# Clean up lock if we are killed.
+# If killed by systemd, like $(systemctl stop restic), then it kills the whole cgroup and all it's subprocesses.
+# However if we kill this script ourselves, we need this trap that kills all subprocesses manually.
+exit_hook() {
+	echo "In exit_hook(), being killed" >&2
+	jobs -p | xargs kill
+	restic unlock
+}
+trap exit_hook INT TERM
+
+# Assert that all needed environment variables are set.
+assert_envvars() {
+	local varnames=("$@")
+	for varname in "${varnames[@]}"; do
+		if [ -z ${!varname+x} ]; then
+			printf "%s must be set for this script to work.\n\nDid you forget to source a /etc/restic/*.env profile in the current shell before executing this script?\n" "$varname" >&2
+			exit 1
+		fi
+	done
+}
+assert_envvars \
+	B2_ACCOUNT_ID B2_ACCOUNT_KEY B2_CONNECTIONS \
+	RESTIC_PASSWORD_FILE RESTIC_REPOSITORY RESTIC_VERBOSITY_LEVEL
+
+
+# Remove locks from other stale processes to keep the automated backup running.
+# NOTE nope, don't unlock like restic_backup.sh. restic_backup.sh should take precedence over this script.
+#restic unlock &
+#wait $!
+
+# Check repository for errors.
+restic check \
+	--option b2.connections="$B2_CONNECTIONS" \
+	--verbose="$RESTIC_VERBOSITY_LEVEL" &
+wait $!