udhcpc: simplify manual renew (make it more similar to automatic one)

function                                             old     new   delta
udhcp_run_script                                     669     673      +4
udhcpc_main                                         2414    2401     -13
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(add/remove: 0/0 grow/shrink: 1/1 up/down: 4/-13)              Total: -9 bytes

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