zgenhostid: accept hostid arguments equal to zero.

A common usage pattern for zgenhostid, including in the ZFS dracut
module, is running it as:

  zgenhostid $(hostid)

However, zgenhostid only accepted hostid arguments greater than 0, which
meant that, when the output of hostid(1) was "00000000", zgenhostid
would error out, even though 0 is a possible return value for the
gethostid(3) function used by hostid(1):

- On current musl libc, gethostid(3) is a stub that always returns 0.
- On glibc, gethostid(3) will return 0 if /etc/hostid exists but is
  smaller than 4 bytes.

In these cases, it makes more sense for zgenhostid to treat a value of 0
as other parts of the zfs codebase do, meaning that a hostid value
couldn't be determined; therefore, it should attempt to generate a
random value to write into /etc/hostid.

The manpage and usage output have been updated to reflect this.

Whitespace has also been fixed in the usage output.

Reviewed-by: Brian Behlendorf <[email protected]>
Reviewed-by: Georgy Yakovlev <[email protected]>
Reviewed-by: Andrew J. Hesford <[email protected]>
Signed-off-by: Érico Rolim <[email protected]>
Closes #11174
Closes #11189

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