pgsql: Remove direct inclusions of as well as configure test

Log Message:
-----------
Remove direct inclusions of <com_err.h> as well as configure test for
its presence.  This amounts to desupporting Kerberos 5 releases 1.0.*,
which is small loss, and simplifies use of our Kerberos code on platforms
with Red-Hat-style include file layouts.  Per gripe from John Gray and
followup discussion.

Modified Files:
--------------
    pgsql:
        configure (r1.417 -> r1.418)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/configure.diff?r1=1.417&r2=1.418)
        configure.in (r1.391 -> r1.392)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/configure.in.diff?r1=1.391&r2=1.392)
    pgsql/src/backend/libpq:
        auth.c (r1.119 -> r1.120)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/libpq/auth.c.diff?r1=1.119&r2=1.120)
    pgsql/src/interfaces/libpq:
        fe-auth.c (r1.95 -> r1.96)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/interfaces/libpq/fe-auth.c.diff?r1=1.95&r2=1.96)