getops: improve getopt-parse.bash example

getops: improve getopt-parse.bash example

Use correct names of example scripts in the script.  Remove use of
backticks, they require quoting that makes the example harder to follow.
Split one-liners to one-command-at-a-time expressions.  Add continue keyword
to avoid additional case statement matching.  Be strict with quoting.

Reported-by: Benno Schulenberg <bensberg@justemail.net>
Signed-off-by: Sami Kerola <kerolasa@iki.fi>