udhcpc: support option 0x79 (static routes) (part of bug 341)

udhcpc: support option 0x79 (static routes) (part of bug 341)

function                                             old     new   delta
xmalloc_optname_optval                                 -     583    +583
dhcp_option_strings                                  258     266      +8
dhcp_options                                          70      72      +2
len_of_option_as_string                               11      12      +1
dhcp_option_lengths                                   11      12      +1
udhcp_run_script                                    1187     669    -518
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(add/remove: 1/0 grow/shrink: 4/1 up/down: 595/-518)           Total: 77 bytes

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