xpipe: introduce (saves ~170 bytes)

udhcp/signalpipe.c: use pipe instead of socketpair.

archival/libunarchive/open_transformer.c

@@ -17,9 +17,7 @@ int open_transformer(int src_fd,
 	int fd_pipe[2];
 	int pid;
 
-	if (pipe(fd_pipe) != 0) {
-		bb_perror_msg_and_die("can't create pipe");
-	}
+	xpipe(fd_pipe);
 
 	pid = fork();
 	if (pid == -1) {

archival/tar.c

@@ -507,8 +507,8 @@ static int writeTarFile(const int tar_fd, const int verboseFlag,
 		volatile int vfork_exec_errno = 0;
 		const char *zip_exec = (gzip == 1) ? "gzip" : "bzip2";
 
-		if (pipe(gzipDataPipe) < 0 || pipe(gzipStatusPipe) < 0)
-			bb_perror_msg_and_die("pipe");
+		xpipe(gzipDataPipe);
+		xpipe(gzipStatusPipe);
 
 		signal(SIGPIPE, SIG_IGN); /* we only want EPIPE on errors */