Import mknod from ubase

Although mknod is not a POSIX tool, it is widely available on nearly
all UNIX-like systems. It also can be implemented portably apart
from use of the makedev macros, which is already a requirement of
a couple other tools in sbase.

While we're at it, fix a few bugs:
- Include sys/sysmacros.h if makedev was not defined by sys/types.h
- The default mode should respect the user's umask, rather than
  assuming it is 022.
- Clear the umask when -m is specified explicitly so that nodes can
  be created with permissions wider than the user's umask.
- Utilize parsemode from libutil to support symbolic mode strings.

mknod.c

@@ -0,0 +1,48 @@
+/* See LICENSE file for copyright and license details. */
+#include <sys/stat.h>
+#include <sys/types.h>
+#ifndef makedev
+#include <sys/sysmacros.h>
+#endif
+
+#include <fcntl.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <unistd.h>
+
+#include "util.h"
+
+static void
+usage(void)
+{
+	eprintf("usage: %s [-m mode] name type major minor\n", argv0);
+}
+
+int
+main(int argc, char *argv[])
+{
+	mode_t type, mode = 0666;
+	dev_t dev;
+
+	ARGBEGIN {
+	case 'm':
+		mode = parsemode(EARGF(usage()), mode, umask(0));
+		break;
+	default:
+		usage();
+	} ARGEND;
+
+	if (argc != 4)
+		usage();
+
+	if (strlen(argv[1]) != 1 || !strchr("ucb", argv[1][0]))
+		eprintf("invalid type '%s'\n", argv[1]);
+	type = (argv[1][0] == 'b') ? S_IFBLK : S_IFCHR;
+
+	dev = makedev(estrtonum(argv[2], 0, LLONG_MAX), estrtonum(argv[3], 0, LLONG_MAX));
+
+	if (mknod(argv[0], type|mode, dev) == -1)
+		eprintf("mknod %s:", argv[0]);
+	return 0;
+}