Update generate_manifest to support AUTO_MASKTABLE

The introduction of AUTO_MASKTABLE caused errors in
generate_manifest.sh, which assumed that mask table handling was always
specified in parameters ending in MASKTABLE.  The command has been
modified to assume that it equals MASKTABLE.

This reveals two points of error:

* This should have been detected when the PR containing AUTO_MASKTABLE
  was added to MOM6-examples.  Unfortunately, it happily runs an empty
  manifest and reports success.

* generate_manifest.sh detected an error (admittedly an absurd
  nonintuitive one) but did not signal this error to the Makefile.

  Unfortunately, there does not appear to be a practical way to signal
  an error from within a flow control block such as if-else-fi.

Hopefully we are replacing this system anyway, so we will simply accept
this patch and move on.

tools/MRS/generate_manifest.sh

@@ -30,7 +30,7 @@ do
   test -f $layout_file && \
     npi=$(egrep "^NIPROC = " $layout_file | awk '{print $3}') &&
     npj=$(egrep "^NJPROC = " $layout_file | awk '{print $3}') &&
-    masktable=($(grep MASKTABLE $layout_file | grep -v 'MASKTABLE = "MOM_mask_table"' | sed 's/MASKTABLE = "[a-zA-Z_]*\.//;s/".*//;s/[\.x]/ /g'))
+    masktable=($(grep "^ *MASKTABLE" $layout_file | grep -v 'MASKTABLE = "MOM_mask_table"' | sed 's/MASKTABLE = "[a-zA-Z_]*\.//;s/".*//;s/[\.x]/ /g'))
     if [[ "${#masktable}" -gt "0" ]]; then
       npes=$((${masktable[1]}*${masktable[2]}-${masktable[0]}))
     else