S9 LIB  (search-path string1 string2)  ==>  string | #f

Search the Unix search path STRING2 for the executable STRING1. Return
the full path of the first executable found or #F if not executable
named STRING1 can be found in the given path.
STRING2 is a colon-separated list of paths, e.g.:

        "/bin:/usr/bin:/usr/local/bin"

SEARCH-PATH uses ACCESS with mode ACCESS-X-OK to check whether a
file is executable.

(search-path "vi" "/bin:/usr/bin")  ==>  "/usr/bin/vi"
