config/dracut/90zfs: handle cases where hostid(1) returns all zeros

On systems with musl libc, hostid(1) always prints "00000000", which
will cause improper behavior when the 90zfs module is configured in a
dracut initramfs. Work around this by copying the host /etc/hostid if
the file exists, and otherwise only write /etc/hostid if hostid(1)
returns something meaningful. This avoids zgenhostid creating a random
/etc/hostid for the initramfs, which could lead to errors when trying to
import the pool if spl_hostid isn't defined in the kernel command line.

Furthermore, tag the /etc/hostid file as hostonly, since it is system
specific and shouldn't be taken into account when trying to use an
initramfs generated in one system to boot into a different system.

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

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