mkswap: remove unnecessary usage output

The usage output changes too often making unnecessarily expected output
in test unstable.  Besides in this case the usage ouput does not even
help user, because invalid swap target size error cannot be fixed by
specificing an option.

Signed-off-by: Sami Kerola <kerolasa@iki.fi>
tests: check swaplabel, and perform more swapon boundary tests

Signed-off-by: Sami Kerola <kerolasa@iki.fi>
build-sys: sort gitignore files

Use 'LC_ALL=C sort -fu' order for all lists.

Signed-off-by: Sami Kerola <kerolasa@iki.fi>
docs: add a new command boilerplate code

Having howtos is good, having an working code that can be used as-is to
kickstart a write of a new command is better.

Signed-off-by: Sami Kerola <kerolasa@iki.fi>