Convert a symbolic mode string specification to an equivalent number.
RIGHTS is the symbolic mode spec, it should match "([+=-][rwxXstugo]*)+".
WHO-MASK is the bit-mask specifying the category of users to which to
apply the access permissions. See `file-modes-char-to-who'.
FROM (or 0 if nil) gives the mode bits on which to base permissions if
RIGHTS request to add, remove, or set permissions based on existing ones, as in "og+rX-w".