check: fix warnings from shellcheck 0.8.0

And make errors gcc-style, so emacs can jump through the automatically.

```
In devtools/reduce-includes.sh line 21:
	    echo -n "-$LINE"
                 ^-- SC3037 (warning): In POSIX sh, echo flags are undefined.


In devtools/reduce-includes.sh line 25:
	    echo -n "."
                 ^-- SC3037 (warning): In POSIX sh, echo flags are undefined.


In tools/rel.sh line 6:
prefix=$(printf '%s\n' "${from#$common}" | sed 's@[^/][^/]*@..@g')
                               ^-----^ SC2295 (info): Expansions inside ${..} need to be quoted separately, otherwise they match as patterns.

Did you mean: 
prefix=$(printf '%s\n' "${from#"$common"}" | sed 's@[^/][^/]*@..@g')


In tools/rel.sh line 7:
printf '%s\n' "$prefix/${to#$common}"
                            ^-----^ SC2295 (info): Expansions inside ${..} need to be quoted separately, otherwise they match as patterns.

Did you mean: 
printf '%s\n' "$prefix/${to#"$common"}"

For more information:
  https://www.shellcheck.net/wiki/SC3037 -- In POSIX sh, echo flags are undef...
  https://www.shellcheck.net/wiki/SC2295 -- Expansions inside ${..} need to b...
make: *** [Makefile:553: check-shellcheck] Error 123
```

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

Makefile

@@ -550,7 +550,7 @@ check-cppcheck: .cppcheck-suppress
 	@trap 'rm -f .cppcheck-suppress' 0; git ls-files -- "*.c" "*.h" | grep -vE '^ccan/' | xargs cppcheck  ${CPPCHECK_OPTS}
 
 check-shellcheck:
-	@git ls-files -- "*.sh" | xargs shellcheck
+	@git ls-files -- "*.sh" | xargs shellcheck -f gcc
 
 check-setup_locale:
 	@tools/check-setup_locale.sh

devtools/reduce-includes.sh

@@ -17,12 +17,11 @@ for file; do
 	grep -F -v "$LINE" "$file" > "$file".c
 
 	if $CCMD /tmp/out.$$.o "$file".c 2>/dev/null; then
-	    # shellcheck disable=SC2039
-	    echo -n "-$LINE"
+	    printf "%s" "-$LINE"
 	    mv "$file".c "$file"
 	else
-	    # shellcheck disable=SC2039
-	    echo -n "."
+	    # shellcheck disable=SC2039,SC3037
+	    printf "."
 	    rm -f "$file".c
 	    i=$((i + 1))
 	fi

tools/rel.sh

@@ -3,5 +3,5 @@
 from=${1}
 to=${2}
 common=$(printf '%s\n%s' "${from}" "${to}" | sed 'N;s/\(.*\).*\n\1.*$/\1/' | sed 's@/[^/]*$@/@')
-prefix=$(printf '%s\n' "${from#$common}" | sed 's@[^/][^/]*@..@g')
-printf '%s\n' "$prefix/${to#$common}"
+prefix=$(printf '%s\n' "${from#"$common"}" | sed 's@[^/][^/]*@..@g')
+printf '%s\n' "$prefix/${to#"$common"}"