udhcpc: rename server_addr to server_id, fix id comparison if it's not specified

Even though it is _meant to be_ an IP address, in the wild servers sometimes
give bogus server ids, like 1.1.1.1

function                                             old     new   delta
udhcpc_main                                         2551    2542      -9

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
1 file changed