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

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