Another minor tweak to psql startup: set the default values for the PROMPT1,

Another minor tweak to psql startup: set the default values for the PROMPT1,
PROMPT2 and PROMPT3 variables before we read any of the settings specified
via the user on the command-line or in psqlrc, so that the latter can
override the former. Per original patch from Ingo van Lil, simpler fix
suggested by Tom Lane.