Calculate the size of the backing store file.

Calculate the size of the backing store file.

A size can be specified in absolute terms (suffix: k, m, g, t supported),
but also as a percentage of the filesystems free space (suffix '%').

If the specified size is larger than an off_t can cope, we bisect
repeatedly until it can.

If the size exceeds the available space of the filesystem, we truncate
to 80% of the free space.

Then round down to an integral number of blocks, sized by the larger
of the filesystem blocksize and the pagesize.

This was tricker than I'd expected...

git-svn-id: svn+ssh://projects.linpro.no/svn/varnish/trunk@175 d4fa192b-c00b-0410-8231-f00ffab90ce4