psql: Add quit/help behavior/hint, for other tool portability

psql:  Add quit/help behavior/hint, for other tool portability

Issuing 'quit'/'exit' in an empty psql buffer exits psql.  Issuing
'quit'/'exit' in a non-empty psql buffer alone on a line with no prefix
whitespace issues a hint on how to exit.

Also add similar 'help' hints for 'help' in a non-empty psql buffer.

Reported-by: Everaldo Canuto
Discussion: https://postgr.es/m/flat/CALVFHFb-C_5_94hueWg6Dd0zu7TfbpT7hzsh9Zf0DEDOSaAnfA%40mail.gmail.com

Author: original author Robert Haas, modified by me