dhcp: get rid of static data signal_pipe

dhcp: get rid of static data signal_pipe

function                                             old     new   delta
udhcp_sp_setup                                        65     110     +45
udhcp_sp_fd_set                                       60      59      -1
udhcpd_main                                         1442    1437      -5
udhcpc_main                                         2684    2679      -5
signal_pipe                                            8       -      -8
packed_usage                                       33292   33284      -8
------------------------------------------------------------------------------
(add/remove: 0/1 grow/shrink: 1/4 up/down: 45/-27)             Total: 18 bytes
   text	   data	    bss	    dec	    hex	filename
 952746	    481	   7296	 960523	  ea80b	busybox_old
 952768	    481	   7288	 960537	  ea819	busybox_unstripped

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