Minor improvements and enforce some standards

Command '[' uses '-n' by default:
* Never use '[ -n "$example" ]'
* Always use '[ "$example" ]'
This improves readabiity.

Use only '-e' to test for file existence.

Try to use simple tests, not 'not not X' (double negative) tests.
Example:
* [ "$EASYRSA_RAND_SERIAL != "no" ] = Replace with ' = "yes" '

Use 'shift' cleanly, immediately after assignment.

Improve/correct comments and user messages.

Signed-off-by: Richard T Bonhomme <[email protected]>