improve install_files for binaries

[rurban]

Currently the logic to install installables is not satisfactory and leads too easily to hard to detect errors.

There's no error when a installable_ version is missing.

Currently:
First the ordinary binaries are installed, then in a second step those are overwritten by the installable_* versions.

Todo:

• For all binaries with matching installable_* binary, copy only the installable_ binary and do not attempt to copy the build_specific variant at all. This is too fragile.
• print a warning if a matching bin installable_ version is not found in the MANIFEST files (such as scripts)
• fail with an error if the installable is missing or could not be copied.

[allison]

Patch rejected. All that's needed to prevent the non-installable versions of the binaries from being installed and then overwritten is to remove the "[main]bin" flag from them in MANIFEST.generated. Done in r38393.

You can't have an error if X file isn't found in the manifest, because the manifest *is* the list of files to install (you'd be comparing the manifest to itself, which will always be true).