pgsql: Fix up canonicalize_path to do the right thing in all cases (I

Log Message:
-----------
Fix up canonicalize_path to do the right thing in all cases (I think ...
this was harder than it seemed at first glance). Also push code for
checking for ".." in file names into path.c where it belongs.

Modified Files:
--------------
pgsql/src/backend/utils/adt:
genfile.c (r1.2 -> r1.3)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/genfile.c.diff?r1=1.2&r2=1.3)
pgsql/src/include:
port.h (r1.80 -> r1.81)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/port.h.diff?r1=1.80&r2=1.81)
pgsql/src/port:
path.c (r1.56 -> r1.57)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/port/path.c.diff?r1=1.56&r2=1.57)