Make a symbolic link to FILENAME, named LINKNAME.
Both args must be strings.
Signals a `file-already-exists' error if a file LINKNAME already exists
unless optional third argument OK-IF-ALREADY-EXISTS is non-nil.
A number as third arg means request confirmation if LINKNAME already exists.
This happens for interactive use with M-x.
(fn FILENAME LINKNAME &optional OK-IF-ALREADY-EXISTS)