Add --dataalignment to pvcreate to specify alignment of data area. (mbroz)

Add --dataalignment to pvcreate to specify alignment of data area.  (mbroz)

This patch is not fully tested and leaves some related bugs unfixed.

Intended behaviour of the code now:

  pe_start in the lvm2 format PV label header is set only by pvcreate (or
vgconvert -M2) and then preserved in *all* operations thereafter.

  In some specialist cases, after the PV is added to a VG, the pe_start
field in the VG metadata may hold a different value and if so, it
overrides the other one for as long as the PV is in such a VG.

  Currently, the field storing the size of the data area in the PV label
header always holds 0.  As it only has meaning in the context of a
volume group, it is calculated whenever the PV is added to a VG (and can
be derived from extent_size and pe_count in the VG metadata).