Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Factor out all the .SHELL parsing related stuff into its own file and
rename the function to be consistent with the naming scheme in the rest
of make. No functional changes.

Obtained from: DragonFlyBSD (idea and most of shell.h)