SSL indicator in psql prompt

I like how browsers show a little lock in the address bar depending on
whether SSL is in use.  This could be useful in psql as well.  Here is a
prototype patch.

Example:

Put this in .psqlrc:

\set PROMPT1 '%s%/%R%# '

$ psql test
psql (9.6devel)
Type "help" for help.

🔒test=#

Without SSL:

🃏test=#

Comments?