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.

README

@@ -84,6 +84,7 @@ The following tools are implemented:
 0=*|x md5sum          .
 0=*|o mkdir           .
 0=*|o mkfifo          .
+0=*|x mknod           .
 0=*|x mktemp          .
 0=*|o mv              (-i)
 0=*|o nice            .