repart: adjust "partno" column

repart: adjust "partno" column

In a typical output from systemd-repart, the output is very wide any any wasted
space is bad because it pushes the interesting information even further to the
right. We usually need at most one or two digits to express the partition
numbers, so let's shorten the title of the column to effectively remove two
columns in the output.

In JSON output, the old field name is retained. This follows the pattern
already used for field "drop-in_files".

Also right-align the columns with numbers always to the right. I doesn't make
sense to align the columns which are only used for JSON output, so stop setting
alignment for those.