Function: file-modes-symbolic-to-number

Convert symbolic file modes to numeric file modes.
MODES is the string to convert, it should match
"[ugoa]*([+-=][rwxXstugo]*)+,...".
See Info node `(coreutils)File permissions' for more information on this
notation.
FROM (or 0 if nil) gives the mode bits on which to base permissions if
MODES request to add, remove, or set permissions based on existing ones, as in "og+rX-w".