man/*sh: fix shellcheck warnings, add to shellcheck

Now the only remaining file that needs shellcheck warnings to be fixed
is bash-completion. Note that in Makefile's TODO.

Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

man/md2man-all.sh

@@ -2,7 +2,7 @@
 set -e
 
 # get into this script's directory
-cd "$(dirname "$(readlink -f "$BASH_SOURCE")")"
+cd "$(dirname "$(readlink -f "${BASH_SOURCE[0]}")")"
 
 [ "$1" = '-q' ] || {
 	set -x
@@ -18,7 +18,7 @@ for FILE in *.md; do
 	base="$(basename "$FILE")"
 	name="${base%.md}"
 	num="${name##*.}"
-	if [ -z "$num" -o "$name" = "$num" ]; then
+	if [ -z "$num" ] || [ "$name" = "$num" ]; then
 		# skip files that aren't of the format xxxx.N.md (like README.md)
 		continue
 	fi