df: fix alignment of columns

df: fix alignment of columns

* src/df.c (alloc_table_row): A new function to allocate storage
for a row of strings.
(print_table): A new function to interate over all stored strings in
the table, and apply alignment honoring the max width of each column.
(get_header): Renamed from print_header, and adjusted accordingly.
(get_dev): Renamed from show_dev.  Also we no longer wrap longer
device names over two lines, which can be an unexpected issue for
scripts parsing the output from df.
(get_disk): s/show_/get_/
(get_point): Likewise.
(get_entry): Likewise.
(get_all_entries): Likewise.
* NEWS: Mention the change.