pgsql: Fix rmtree() so that it keeps going after failure to remove any

Log Message:
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Fix rmtree() so that it keeps going after failure to remove any individual
file; the idea is that we should clean up as much as we can, even if there's
some problem removing one file. Make the error messages a bit less misleading,
too. In passing, const-ify function arguments.

Tags:
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REL8_3_STABLE

Modified Files:
--------------
pgsql/src/backend/commands:
dbcommands.c (r1.204.2.2 -> r1.204.2.3)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/dbcommands.c?r1=1.204.2.2&r2=1.204.2.3)
pgsql/src/include:
port.h (r1.116.2.4 -> r1.116.2.5)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/port.h?r1=1.116.2.4&r2=1.116.2.5)
pgsql/src/port:
dirmod.c (r1.51.2.3 -> r1.51.2.4)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/port/dirmod.c?r1=1.51.2.3&r2=1.51.2.4)