Pull request: install check

Merge in DNS/adguard-home from 2453-install-check to master

Closes #2453.

Squashed commit of the following:

commit b3123d7171ff5d1e00d8bcbb5cbe7fcf7a142a3c
Author: Eugene Burkov <e.burkov@adguard.com>
Date:   Fri Dec 18 14:00:26 2020 +0300

    all: fix quotes

commit 27e17f9543250d912dd559c9ba2e01e35636551f
Author: Eugene Burkov <e.burkov@adguard.com>
Date:   Fri Dec 18 13:34:00 2020 +0300

    all: improve install script

commit e9a927ffabc04dcd223bfc0b3b2541c7d5b96b61
Author: Eugene Burkov <e.burkov@adguard.com>
Date:   Fri Dec 18 13:22:05 2020 +0300

    all: add directory emptiness check

HACKING.md

@@ -163,12 +163,15 @@ The rules are mostly sorted in the alphabetical order.
 
 ##  Shell Scripting
 
- *  Avoid bashisms, prefer *POSIX* features only.
+ *  Avoid bashisms and GNUisms, prefer *POSIX* features only.
 
  *  Prefer `'raw strings'` to `"double quoted strings"` whenever possible.
 
  *  Put spaces within `$( cmd )`, `$(( expr ))`, and `{ cmd; }`.
 
+ *  Put utility flags in the ASCII order and **don't** group them together.  For
+    example, `ls -1 -A -q`.
+
  *  `snake_case`, not `camelCase`.
 
  *  Use `set -e -f -u` and also `set -x` in verbose mode.
@@ -176,6 +179,24 @@ The rules are mostly sorted in the alphabetical order.
  *  Use the `"$var"` form instead of the `$var` form, unless word splitting is
     required.
 
+ *  When concatenating, always use the form with curly braces to prevent
+    accidental bad variable names.  That is, `"${var}_tmp.txt"` and **not**
+    `"$var_tmp.txt"`.  The latter will try to lookup variable `var_tmp`.
+
+ *  When concatenating, surround the whole string with quotes.  That is, use
+    this:
+
+    ```sh
+    dir="${TOP_DIR}/sub"
+    ```
+
+    And **not** this:
+
+    ```sh
+    # Bad!
+    dir="${TOP_DIR}"/sub
+    ```
+
 ##  Text, Including Comments
 
  *  End sentences with appropriate punctuation.