ZNETBOOT release 20180226

This is ZNETBOOT release 20180226.
In this release, ZNETBOOT EXEC works, supporting several Linux distributions.

The ZNETBOOT automation for the centerpiece distribution is not quite ready.

A progress reporter has been added so that one can know a long-running
download has not stalled.

Useful ZNETBOOT tags include:

ZNETBOOT_KERNEL=
ZNETBOOT_INITRD=
ZNETBOOT_TOFILE=
ZNETBOOT_PROGRESS=

Assets 4

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