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

etc/cron.d/restic

@@ -1,5 +1,5 @@
 SHELL=/bin/sh
-PATH=/etc:/bin:/sbin:/usr/bin:/usr/sbin:/usr/local/bin/:/usr/local/sbin/
+PATH=/etc:/bin:/sbin:/usr/bin:/usr/sbin:/usr/local/bin/:$INSTALL_PREFIX/sbin/
 # Order of crontab fields
 # minute        hour    mday    month   wday    command
 # Reference: https://www.freebsd.org/doc/handbook/configtuning-cron.html