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

Log Message:
-----------
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.

Modified Files:
--------------
    pgsql/src/backend/commands:
        dbcommands.c (r1.207 -> r1.208)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/dbcommands.c?r1=1.207&r2=1.208)
    pgsql/src/include:
        port.h (r1.121 -> r1.122)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/port.h?r1=1.121&r2=1.122)
    pgsql/src/port:
        dirmod.c (r1.54 -> r1.55)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/port/dirmod.c?r1=1.54&r2=1.55)