style: Normalize variable expansions to ${var} across the repo

Replace bare $var with ${var} everywhere it was still used, matching
the convention already established in rc.d/prompt and rc.d/history.
Positional parameters ($1, $@, $#) and special variables ($?, $$, $-)
are left as-is, consistent with existing usage in _helpers.bash.

rc.d/broot is auto-generated by the broot binary and will likely be
overwritten with its original formatting on the next broot update.

rc.d/cgroups

@@ -1,5 +1,5 @@
 # Use cgroups
-if [ "$PS1" ] ; then
+if [ "${PS1}" ] ; then
 
 	if [ -d /sys/fs/cgroup ] ; then
 		cdir=/sys/fs/cgroup
@@ -7,10 +7,10 @@ if [ "$PS1" ] ; then
 		cdir=/dev/cgroup
 	fi
 
-	if [ -d "$cdir/user" ]; then
-		mkdir -p -m 0700 $cdir/user/$$ > /dev/null 2>&1
-		/bin/echo $$ > $cdir/user/$$/tasks
-		/bin/echo '1' > $cdir/user/$$/notify_on_release
+	if [ -d "${cdir}/user" ]; then
+		mkdir -p -m 0700 "${cdir}/user/$$" > /dev/null 2>&1
+		/bin/echo $$ > "${cdir}/user/$$/tasks"
+		/bin/echo '1' > "${cdir}/user/$$/notify_on_release"
 		unset -v cdir
 	fi