scripts: dhcp/dhcpv6: handling of invalid client ID values

scripts: dhcp/dhcpv6: handling of invalid client ID values

Verify and clean up client ID and global DUID config values before use, in
order to prevent DHCP client malfunction and loss of IPv4 and/or IPv6
connectivity.

- Accept common separators in the string (colon, dash, spaces/tabs, lf)
- Ignore invalid values (non-hex or odd-numbered length)
- Log a warning for invalid settings

The fall back mechanism is as follows:

If a (user-configured) network.<ifname>.clientid setting is found invalid,
ignore and fall back to using network.globals.dhcp_default_duid. In case
that's also invalid (e.g. has been tampered with), don't pass a client
ID to updhc/odhcpc. In that case, udhcpc/odhcpc will use their defaults,
which currently is the i/f MAC address resp. the i/f DUID-LL.

Only error handling is introduced, no behavior change for valid settings.

Signed-off-by: Shine <4c.fce2@proton.me>
Link: https://github.com/openwrt/openwrt/pull/23662
Signed-off-by: Robert Marko <robimarko@gmail.com>