setup-env.sh: make setup scripts work with set -u

- Add set -u to the setup-env.sh test script

- Refactor lines in setup-env.sh that tested potentially undefined
  variables to use the `[ -z ${var+x} ]` construct

share/spack/qa/setup-env-test.sh

@@ -160,6 +160,9 @@ unuse() {
 # Make sure no environment is active
 unset SPACK_ENV
 
+# fail with undefined variables
+set -u
+
 # Source setup-env.sh before tests
 .  share/spack/setup-env.sh
 

share/spack/setup-env.sh

@@ -47,8 +47,9 @@ spack() {
     fi
 
     # accumulate initial flags for main spack command
+    # trnaslation for humans: "while $1 is set and $1 starts with -"
     _sp_flags=""
-    while [ "${1#-}" != "${1}" ]; do
+    while [ ! -z ${1+x} ] && [ "${1#-}" != "${1}" ]; do
         _sp_flags="$_sp_flags $1"
         shift
     done
@@ -63,7 +64,7 @@ spack() {
     fi
 
     _sp_subcommand=""
-    if [ -n "$1" ]; then
+    if [ ! -z ${1+x} ]; then
         _sp_subcommand="$1"
         shift
     fi
@@ -102,7 +103,7 @@ spack() {
                 case $_sp_arg in
                     activate)
                         _a="$@"
-                        if [ -z "$1" ] || \
+                        if [ -z ${1+x} ] || \
                            [ "${_a#*--sh}" != "$_a" ] || \
                            [ "${_a#*--csh}" != "$_a" ] || \
                            [ "${_a#*-h}" != "$_a" ];