udhcpd: remove five more options which do not make sense or not supported

udhcpd: remove five more options which do not make sense or not supported

requestip, vendorclass, clientid are client-side variables,
          they do not make sense as udhcpd opts
dhcptype  is the packet type (not interesting, it's always 5)
userclass needs parser support in order to work

function                                             old     new   delta
dhcp_options                                          68      66      -2
read_opt                                             865     859      -6
dhcp_option_strings                                  253     203     -50
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(add/remove: 0/0 grow/shrink: 0/3 up/down: 0/-58)             Total: -58 bytes

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