Mark sslkeylogfile as Debug option

Mark sslkeylogfile as Debug option

Mark the sslkeylogile option as "D" debug as this truly is a debug
option, and it will allow postgres_fdw et.al to filter it out as
well.  Also update the display length to match that for an ssl key
as they are both filename based inputs.

Author: Daniel Gustafsson <daniel@yesql.se>
Reported-by: Jacob Champion <jacob.champion@enterprisedb.com>
Discussion: https://postgr.es/m/CAOYmi+=5GyBKpu7bU4D_xkAnYJTj=rMzGaUvHO99-DpNG_YKcw@mail.gmail.com