Move code to initialize and search %load-path from ice-9 to C

Move code to initialize and search %load-path from ice-9 to C
code, so we can use the load-path to find the ice-9 boot code;
this makes it easier to run Guile without installing it.  See
corresponding changes in guile/Makefile.in.
* feature.c: Move stuff concerned with the load path to load.c.
(scm_compiled_library_path): Deleted.
Don't #include libpath.h here.
* feature.h: Don't mention scm_compiled_library_path.
* load.c: #include "libpath.h" here, as well as <sys/types.h>,
<sys/stat.h>, and <unistd.h> (if present).
(R_OK): #define if the system hasn't deigned to.
(scm_loc_load_path): New variable.
(scm_init_load_path, scm_sys_search_load_path,
scm_sys_try_load_path): New functions.
(scm_init_load): Initialize scm_loc_load_path to point to the
value cell of the Scheme %load-path variable.
* load.h: Add declarations for scm_sys_search_load_path,
scm_sys_try_load_path.
* init.c: Call scm_init_load_path.
* Makefile.in (feature.o, load.o): Dependencies updated.

* load.c, load.h: Rewrite using PROTO macro.