Search for FILENAME through PATH.
If found, return the absolute file name of FILENAME, with its suffixes;
otherwise return nil.
PATH should be a list of directories to look in, like the lists in
`exec-path' or `load-path'.
If SUFFIXES is non-nil, it should be a list of suffixes to append to
file name when searching. If SUFFIXES is nil, it is equivalent to '("").
Use '("/") to disable PATH search, but still try the suffixes in SUFFIXES.
If non-nil, PREDICATE is used instead of `file-readable-p'.
This function will normally skip directories, so if you want it to find
directories, make sure the PREDICATE function returns `dir-ok' for them.
PREDICATE can also be an integer to pass to the `access' system call,
in which case file-name handlers are ignored. This usage is deprecated.
For compatibility, PREDICATE can also be one of the symbols
`executable', `readable', `writable', or `exists', or a list of one or more of those symbols.