netifd: dhcp: suppress udhcpc default vendor class if specified in sendopts

netifd: dhcp: suppress udhcpc default vendor class if specified in sendopts

When DHCP Option 60 is specified via sendopts (hex, decimal, or named
formats), udhcpc sends its default "udhcp <version>" string alongside
the custom value, which causes authentication failures with some ISPs.

This fix detects Option 60 in sendopts and automatically passes -V ""
to udhcpc to suppress the default version string while allowing
multiple user-defined vendor classes.

Supported formats:
- Hexadecimal: 0x3c
- Decimal: 60
- Named: vendor

Fixes: #21242
Signed-off-by: JINLIANG GU <ihipop@gmail.com>
https://github.com/openwrt/openwrt/pull/21450
Signed-off-by: Álvaro Fernández Rojas <noltari@gmail.com>