examples: replace grep command in NM dispatcher script

examples: replace grep command in NM dispatcher script

Some grep implementations detect binary data and return success without
matching whole line. This might be an issue for the DHCPv6 NTP FQDN
check. The GNU grep in the C locale seems to check only for the NUL
character, which cannot be passed in an environment variable, but other
implementations might behave differently and there doesn't seem to be a
portable way to force matching the whole line.

Instead of the grep command, check for invalid characters by comparing
the length of the input passed through "tr -d -c".