mirror/raid: Move 'copy_percent' to common code (mirror.c -> lv_manip.c)

mirror/raid:  Move 'copy_percent' to common code (mirror.c -> lv_manip.c)

The 'copy_percent' function takes the 'extents_copied' field from each
segment in an LV to create the numerator for the ratio that is to
become the copy_percent.  (Otherwise known as the 'sync' percent for
non-pvmove uses, like mirror LVs and RAID LVs.)  This function safely
works on RAID - not just mirrors - so it is better to have it in
lv_manip.c rather than mirror.c.

There's a lot of different functions that do a lot of different things
in lv_manip.c, so I placed the function near a function in lv_manip.c
that it was close to in metadata-exported.h.  Different placement in the
file or a different name for the function may be useful.