po/make_pot: increase portability across shells

Base POSIX leaves the handling of backslahes in the operand
implementation defined. The XSI extension specifies several
escape sequences, like e.g. \n, which shall be transformed
upon printing.
Current make_pot.sh expects XSI behaviour and indeed e.g.
dash’s echo builtin implements this. echo builtins of other common
shells such as bash however, do not (by default). Avoid this portability
pitfall by just using printf at all relevant places. See:
  https://pubs.opengroup.org/onlinepubs/9699919799/utilities/echo.html
  https://pubs.opengroup.org/onlinepubs/9699919799/utilities/printf.html

This also allows us to stop substituting each backslash with four
backslashes before piping to maybe_append. This substituion existed
to safely pass backslahes through maybe_append’s two layers of echo.

Also improve quoting. For consistency prefer single-quote quoting over
shell backslash escapes. QUote the unguarded *.lua to ensure it will
continue to work if there happen to be any lua files in the scripts
working directory.

This supersedes commits from wangqr/Aegisub which adjusted quoting and
echo usage to work with different shells. This is more throughout and
doesn't introduce semi-broken intermediate states.
  0fbcaea871
  4aee271d03
  940181c7bc
(Note the second commit actually didn't manage to achieve the intended
portability, since several echo usages remained and it forgot to adjust
the backslash substitution, which was then fixed in the third commit.)

po/make_pot.sh

@@ -3,17 +3,18 @@ set -e
 
 maybe_append() {
   while read -r msg; do
-    msgfile=$(echo $msg | cut -d'|' -f1)
-    msgline=$(echo $msg | cut -d'|' -f2)
-    msgid=$(echo $msg | cut -d'|' -f3-)
+    msgfile=$(printf '%s' "$msg" | cut -d'|' -f1)
+    msgline=$(printf '%s' "$msg" | cut -d'|' -f2)
+    msgid=$(printf '%s' "$msg" | cut -d'|' -f3-)
 
     if ! grep -Fq "msgid $msgid" aegisub.pot; then
-      echo "\n#: $msgfile:$msgline\nmsgid $msgid\nmsgstr \"\"\n" >> aegisub.pot
+      printf "\n#: %s:%s\nmsgid %s\nmsgstr \"\"\n\n" \
+        "$msgfile" "$msgline" "$msgid" >> aegisub.pot
     fi
   done
 }
 
-find ../src ../src/command -name \*.cpp -o -name \*.h \
+find ../src ../src/command -name '*.cpp' -o -name '*.h' \
   | xgettext --files-from=- -o - --c++ --sort-by-file \
              -k_ -kSTR_MENU -kSTR_DISP -kSTR_HELP -kfmt_tl -kfmt_plural:2,3 -kwxT -kwxPLURAL:1,2 \
   | sed 's/SOME DESCRIPTIVE TITLE./Aegisub 3.2/' \
@@ -35,11 +36,10 @@ grep '"[A-Za-z ]\+" : {' -n ../src/libresrc/default_hotkey.json \
   | sed 's/^\([0-9]\+:\).*\("[^"]\+"\).*$/default_hotkey.json|\1|\2/' \
   | maybe_append
 
-find ../automation -name *.lua \
+find ../automation -name '*.lua' \
   | LC_ALL=C sort \
-  | xargs grep tr\"[^\"]\*\" -o -n \
+  | xargs grep 'tr"[^"]*"' -o -n \
   | sed 's/\(.*\):\([0-9]\+\):tr\(".*"\)/\1|\2|\3/' \
-  | sed 's/\\/\\\\\\\\/g' \
   | maybe_append
 
 xgettext ../packages/desktop/aegisub.desktop.in.in \
@@ -50,7 +50,7 @@ xgettext ../packages/desktop/aegisub.appdata.xml.in.in \
 
 grep '^_[A-Za-z0-9]*=.*' ../packages/win_installer/fragment_strings.iss.in | while read line
 do
-  echo "$line" \
+  printf '%s\n' "$line" \
     | sed 's/[^=]*=\(.*\)/packages\/win_installer\/fragment_strings.iss|1|"\1"/' \
     | maybe_append
 done