udhcp: do not clobber errno by signal handler

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

networking/udhcp/signalpipe.c

@@ -25,9 +25,11 @@ static struct fd_pair signal_pipe;
 
 static void signal_handler(int sig)
 {
+	int sv = errno;
 	unsigned char ch = sig; /* use char, avoid dealing with partial writes */
 	if (write(signal_pipe.wr, &ch, 1) != 1)
 		bb_perror_msg("can't send signal");
+	errno = sv;
 }
 
 /* Call this before doing anything else. Sets up the socket pair