Add line numbers; show and highlight found tabs

This makes the output considerably easier to use: it now highlights (in
red) matched tabs (instead of just listing the filenames), and adds
line numbers to both the tabs check and the space-less if check outputs.

tools/check-style.sh

@@ -9,7 +9,8 @@
 errors=0
 IFS=$'\n'
 found=
-exec 3< <(grep $'\t' include/ tests/ docs/*.rst -rl)
+# The mt=41 sets a red background for matched tabs:
+exec 3< <(GREP_COLORS='mt=41' grep $'\t' include/ tests/ docs/*.rst -rn --color=always)
 while read -u 3 f; do
     if [ -z "$found" ]; then
         echo -e '\e[31m\e[01mError: found tabs instead of spaces in the following files:\e[0m'
@@ -21,7 +22,7 @@ while read -u 3 f; do
 done
 
 found=
-exec 3< <(grep '\<\(if\|for\|while\)(' include/ tests/*.{cpp,py,h} -r --color=always)
+exec 3< <(grep '\<\(if\|for\|while\)(' include/ tests/*.{cpp,py,h} -rn --color=always)
 while read -u 3 line; do
     if [ -z "$found" ]; then
         echo -e '\e[31m\e[01mError: found the following coding style problems:\e[0m'