1.0.12.37: RUN-PROGRAM now uses execvp(3) to search for executables

1.0.12.37: RUN-PROGRAM now uses execvp(3) to search for executables

* RUN-PROGRAM now uses execvp(3) in the child, rather than a Lisp
  function in SBCL, to search for an executable to run.  This makes
  RUN-PROGRAM slightly closer to most other languages' process
  creation facilities.

* The function FIND-EXECUTABLE-IN-SEARCH-PATH has been removed.  A
  note is added to the manual pointing users who need the old,
  idiosyncratic search behavior to look for it it in the CVS history.