treewide: Make still dont* Variables are optional in most cases

Go beyond the obvious setup hooks now, with a bit of sed, with a skipped case:

 - cc-wrapper's `dontlink`, because it already is handled.

Also, in nix files escaping was manually added.

EMP

pkgs/stdenv/cygwin/rebase-i686.sh

@@ -1,7 +1,7 @@
 fixupOutputHooks+=(_cygwinFixAutoImageBase)
 
 _cygwinFixAutoImageBase() {
-    if [ "$dontRebase" == 1 ] || [ ! -d "$prefix" ]; then
+    if [ "${dontRebase-}" == 1 ] || [ ! -d "$prefix" ]; then
         return
     fi
     find "$prefix" -name "*.dll" -type f | while read DLL; do

pkgs/stdenv/cygwin/rebase-x86_64.sh

@@ -1,7 +1,7 @@
 fixupOutputHooks+=(_cygwinFixAutoImageBase)
 
 _cygwinFixAutoImageBase() {
-    if [ "$dontRebase" == 1 ] || [ ! -d "$prefix" ]; then
+    if [ "${dontRebase-}" == 1 ] || [ ! -d "$prefix" ]; then
         return
     fi
     find "$prefix" -name "*.dll" -type f | while read DLL; do