BloombergLP::bdls::FilePermissions::isValidBaseBits

Return true if the specified permissions bit mask consists only of the "base" nine rwx bits (owner/group/others read/write/execute, i.e., bits in k_ALL), and false otherwise. Note that this predicate is stricter than isValid: k_SET_UID, k_SET_GID, and k_STICKY_BIT are rejected.