fast installation via hard-linking

to speed up installation, qbs should be able to create hard links instead of copying files.

to consider:

• symlinks possibly should not be hardlinked - the POSIX standard does not define whether it's even possible to do that (it is on linux)
• for files which are installed as-is, the file attributes of the sources must match the installed permissions. this obviously precludes cross-user hardlink installation.
• cross-device/-partition installs must fall back to copying
• this mode should be the default for automatic installation into a subdirectory of the build dir