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.
