Avoid warning on grep 3.8 in hwclock

Starting with grep version 3.8, the hwclock init script logs warnings
about stray backslashes:

> hwclock | * Setting system clock using the hardware clock [UTC] ...
> hwclock |grep: warning: stray \ before -
> hwclock |grep: warning: stray \ before -

This is caused by the check for existence of the `--noadjfile` argument
in function `get_noadjfile()`.

Replacing the affected logic with an explicit argument denoting the
pattern as such resolves the issue.

Fixes #548

init.d/hwclock.in

@@ -72,7 +72,7 @@ get_noadjfile()
 {
 	if ! yesno $clock_adjfile; then
 		# Some implementations don't handle adjustments
-		if LC_ALL=C hwclock --help 2>&1 | grep -q "\-\-noadjfile"; then
+		if LC_ALL=C hwclock --help 2>&1 | grep -q -e "--noadjfile"; then
 			echo --noadjfile
 		fi
 	fi