Merging scratch/no-purespace to remove unexec and purespace

[Stefan Kangas <stefankangas@gmail.com>]

In August, we decided to remove the unexec build along with purespace.
The scratch/no-purespace branch removes both, and has been rebased on
top of a recent master.

We, the maintainers, believe that the scratch/no-purespace branch is
now ready to merge, and would appreciate any final feedback, testing,
and code reviews.  Specifically, the branch has been primarily tested
on GNU/Linux and macOS, so testing on other systems would be greatly
appreciated.

Unless we uncover any serious blocking issues, or significant or
unexpected objections from the community, we plan to merge the
branch on February 1, 2025.

During our last discussion, we identified some issues with using
pdumper on some legacy proprietary systems: MS-DOS, Windows 98, and
Solaris 10 Zone.  As we have explained previously, patches are welcome
for these issues, but we do not currently consider them as blockers
for this merge.

Thanks!

Stefan Kangas, on behalf of the Emacs maintainers