Use symbolic manifests for more st_mode munging.

Use S_IRWXO instead of 7.  This should be defined on most platforms,
but define it for any that don't.

Use S_G2O_shift,S_U2O_shift instead of 3,6.  I don't know
of any portable way to derive these.

Signed-off-by: Marcus Watts <mdw@linuxbox.com>
Use strcmp not strncmp.

As suggested by Frank Filz <ffilzlnx@mindspring.com>

The original logic used strncmp matching with the length of the user
provided string.  That meant "ANO" or even "A" would match "ANONYMOUS@",
which seems wrong.  If we're not matching against the prefix, then as
suggested plain strcmp does all we need.

Signed-off-by: Marcus Watts <mdw@linuxbox.com>