dhcpv6: RFC4833 timezones

dhcpv6: RFC4833 timezones

This implements RFC4833 - supplying timezone information to clients that
request them. Both forms are possible, when timezone is configured in
the uci system settings (the luci GUI saves both forms to the config).

e.g.
```
config system
	option zonename 'America/Puerto Rico'
	option timezone 'AST4'
```

There is also an odhcpd flag to disable their use, set in uci dhcp.

```
config odhcpd 'odhcpd'
	option enable_tzdb '0'
```

Once enabled, the options, when requested, are sent:
NEW_POSIX_TIMEZONE 41 // 'AST4'
NEW_TZDB_TIMEZONE 42 // 'America/Puerto_Rico'

Wireshark disassemble of options sent to client:

```
...
Time Zone Database
    Option: Time Zone Database (42)
    Length: 19
    TZ-database: America/Puerto_Rico
Time Zone
    Option: Time Zone (41)
    Length: 4
    Time-zone: AST4
```

Signed-off-by: Paul Donald <[email protected]>
Link: https://github.com/openwrt/odhcpd/pull/284
Signed-off-by: Álvaro Fernández Rojas <[email protected]>