Make the location of the Kerberos server key file run time configurable

Make the location of the Kerberos server key file run time configurable
(rather than compile time). For libpq, even when Kerberos support is
compiled in, the default user name should still fall back to geteuid()
if it can't be determined via the Kerberos system.

A couple of fixes for string type configuration parameters, now that there
is one.