mass rewrite of find parameters to cross-platform style

Fixes #9044, close #9667. Thanks to @taku0 for suggesting this solution.
Now we have no modes starting with `/` or `+`.

Rewrite the `-perm` parameters of find:
 - completely safe: rewrite `/0100` and `+100` to `-0100`,
 - slightly semantics-changing: rewrite `+111` to `-0100`.
I cross-verified the `find` manual pages for Linux, Darwin, FreeBSD.

pkgs/build-support/setup-hooks/patch-shebangs.sh

@@ -18,7 +18,7 @@ patchShebangs() {
     local oldInterpreterLine
     local newInterpreterLine
 
-    find "$dir" -type f -perm /0100 | while read f; do
+    find "$dir" -type f -perm -0100 | while read f; do
         if [ "$(head -1 "$f" | head -c +2)" != '#!' ]; then
             # missing shebang => not a script
             continue