(file-modes-char-to-who, file-modes-char-to-right)

(file-modes-char-to-who, file-modes-char-to-right)
(file-modes-rights-to-number): Auxiliary functions for symbolic to
numeric notation of file modes.
(file-modes-symbolic-to-number): New.  Convert symbolic modes to its
numeric value.
(read-file-modes): New.  Read either an octal value of a file mode or a
symbolic value, and return its numeric value.