scripts: dhcp: option to override preferred Client ID per interface

scripts: dhcp: option to override preferred Client ID per interface

Using UUID-based client IDs for DHCPv4/DHCPv6 with no option of falling
back to hardware IDs (ie. MAC-address or DUID-LL) resp. none at all (IPv4),
is causing regressions in some setups.

Introduce a new setting to override the preferred client ID to be used for
DHCPv4/DHCPv6 on a per-interface basis:

network.<ifname>.sendclientid='auto|global|hardware|none'

- "auto" (default if empty or not present) uses any explicitly defined
  client ID, or falls back to the global DUID and finally to the DUID-LL
  resp. MAC address (ie. identical to before this commit).
- "global" uses the global default DUID, if configured, for DHCPv4 and
  DHCPv6 requests, even if a client ID is explicitly specified for the i/f
- "hardware" will not pass a client ID to udhcpc/odhcp6c, even if a global
  default DUID is configured or an explicit client ID specified, resulting
  in the i/f MAC address resp. type 3 DUID(-LL) to be used
- "none" (IPv4 only) will not add an option tag 61 to DHCPv4 requests at
  all.

Signed-off-by: Shine <4c.fce2@proton.me>
Link: https://github.com/openwrt/openwrt/pull/24107
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>