Update heuristic used for default and detected data alignment.

Update heuristic used for default and detected data alignment.

Add "devices/default_data_alignment" to lvm.conf to control the internal
default that LVM2 uses: 0==64k, 1==1MB, 2==2MB, etc.

If --dataalignment (or lvm.conf's "devices/data_alignment") is specified
then it is always used to align the start of the data area.  This means
the md_chunk_alignment and data_alignment_detection are disabled if set.

(Same now applies to pvcreate --dataalignmentoffset, the specified value
will be used instead of the result from data_alignment_offset_detection)

set_pe_align() still looks to use the determined default alignment
(based on lvm.conf's default_data_alignment) if the default is a
multiple of the MD or topology detected values.