Search for FILENAME through PATH.
Returns the file's name in absolute form, or nil if not found.
If SUFFIXES is non-nil, it should be a list of suffixes to append to
file name when searching.
If non-nil, PREDICATE is used instead of `file-readable-p'.
PREDICATE can also be an integer to pass to the access(2) function,
in which case file-name-handlers are ignored.
This function will normally skip directories, so if you want it to find
directories, make sure the PREDICATE function returns `dir-ok' for them.
(fn FILENAME PATH &optional SUFFIXES PREDICATE)