borg now has the chunks list in every item with content.
due to the symmetric way how borg now deals with hardlinks using
item.hlid, processing gets much simpler.
but some places where borg deals with other "sources" of hardlinks
still need to do some hardlink management:
borg uses the HardLinkManager there now (which is not much more
than a dict, but keeps documentation at one place and avoids some
code duplication we had before).
item.hlid is computed via hardlink_id function.
support hardlinked symlinks, fixes#2379
as we use item.hlid now to group hardlinks together,
there is no conflict with the item.source usage for
symlink targets any more.
2nd+ hardlinks now add to the files count as did the 1st one.
for borg, now all hardlinks are created equal.
so any hardlink item with chunks now adds to the "file" count.
ItemFormatter: support {hlid} instead of {source} for hardlinks
general.rst and man_intro.rst both included usage_general.rst.inc, which resulted in three Sphinx warning "WARNING: duplicate label".
To prevent this we move all sections of usage_general into own include files and add a second usage_general file without the three labels.
