Restore signature of `scm_primitive_load_path ()' as found in 1.8.

Restore signature of `scm_primitive_load_path ()' as found in 1.8.

The incompatibility was introduced by
0fb81f95b0222c5ba49efd3e36cf797df54c0863 ("add exception_on_error
optional arg to primitive-load-path").

* libguile/load.c (scm_primitive_load_path): Change to take 1 rest
  argument.  Interpret the argument as either a file name (C-level
  backward compatibility with 1.8) or an actual argument list.
  (scm_c_primitive_load_path): Update caller.

* libguile/load.h (scm_primitive_load_path): Update accordingly.

* doc/ref/api-evaluation.texi (Loading): Update documentation of
  `primitive-load-path' and `scm_primitive_load_path ()'.