Improve bootstrap-haskell and fix shellcheck warnings

scripts/bootstrap/bootstrap-haskell

@@ -197,12 +197,15 @@ posix_realpath() {
     current_loop=0
     max_loops=50
     mysource=$1
+    # readlink and '[ -h $path ]' behave different wrt '/sbin/' and '/sbin', so we strip it
+    mysource=${mysource%/}
+    [ -z "${mysource}" ] && mysource=$1
 
     while [ -h "${mysource}" ]; do
         current_loop=$((current_loop+1))
         mydir="$( cd -P "$( dirname "${mysource}" )" > /dev/null 2>&1 && pwd )"
         mysource="$(readlink "${mysource}")"
-        [ "${mysource%${mysource#?}}"x != '/x' ] && mysource="${mydir}/${mysource}"
+        [ "${mysource%"${mysource#?}"}"x != '/x' ] && mysource="${mydir%/}/${mysource}"
 
         if [ ${current_loop} -gt ${max_loops} ] ; then
             (>&2 echo "${1}: Too many levels of symbolic links")