build.d: Move install from posix.mak

[MoonlightSentinel]

470c402 fixes install-copy omitting the configuration file (dmd.conf / sc.ini) and extends it for other OS to make it re-useable.
470c402 then renames 'install-copy' to install and adds 'install-copy' as a legacy alias.

TODO (inconsistencies between windows/posix):

• (DMD)/src/posix.mak omits dmd's sources, should it copy them like win32.mak currently does?,
• (DMD)/posix.mak includes docs and samples, should windows have this as well?

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[thewilsonator]

Is this good to go?

[MoonlightSentinel]

The first commit is ready IMO but I would prefer some feedback from @wilzbach and @marler8997 on how to deal with install and install-copy (because they have the same usecase but copy different files).

Could submit the first commit in another PR.

[MoonlightSentinel]

See #10703

[marler8997]

@MoonlightSentinel I think now I'm mostly satisfied with the state of build.d, so I'm less motivated to work on it now. I'll let you handle the install targets and do what you think is best. I don't see any issues with my quick glance at the changes.