Add '-s' option to simply display the number of bytes available for the rdroot
filesystem in the specified kernel.

No behaviour change to existing uses of rdsetroot.

Improved option handling & ok k

Add '-s' option to simply display the number of bytes available for the rdroot
filesystem in the specified kernel.

No behaviour change to existing uses of rdsetroot.

Improved option handling & ok kn@

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Sync usage with SYNOPSIS, mention that -x may use stdout

Feedback from jmc and Crystal Kolipe
OK jmc

insert a missing .El and fix a typo

take elfrdsetroot, clean it up for regular usage, and make it a full
citizen of base.
push from benno, ok mortimer ingo others