Fix for bug #91080: nsmkdir truncates mode if it's somewhat valid but too long

ns/Cns_procreq.c:

Added checks on unmarshalled mode to make sure it isn't an invalid number (no more blind trust on client).

ns/Cns_mkdir.c and ns/Cns_chmod.c:

Removal of silent truncating of specified mode via bitmask and substitution with explicit checking. The library aborts if an invalid mode is found instead of "fixing" the input.

ns/nsmkdir.c:

Added explicit validation of submitted mode. It wouldn't be strictly needed as the server wouldn't allow invalid modes anyway now, but it allows for the printing of a specific error message and avoids useless communications with the nameserver.