Move most of the guts of shell command processing into libguile,

so guile.c can be very small (and eventuallly auto-generated.  (I
mean, generated mechanically, not self-generated.  Hmm.))
* guile.c, script.c, script.h: New source files.
* init.c (scm_boot_guile_1): Call scm_init_script.
* libguile.h: #include "script.h".
* Makefile.am (bin_PROGRAMS, guile_SOURCES, guile_LDADD): New
targets, for new executable.
(libguile_la_SOURCES): Mention script.c.
(modinclude_HEADERS): Add script.h.
* configure.in: Always check for -lm, -lsocket, -lnsl, whether or
not dynamic linking is enabled.  This is because we're generating
executables now.  Move CY_AC_WITH_THREADS call after those, so the
values of cy_cv_threads_libs captures the libs chosen above.
* Makefile.in, configure, aclocal.m4: Regenerated.

libguile/libguile.h

@@ -99,6 +99,7 @@
 #include "libguile/read.h"
 #include "libguile/root.h"
 #include "libguile/scmsigs.h"
+#include "libguile/script.h"
 #include "libguile/sequences.h"
 #include "libguile/simpos.h"
 #include "libguile/snarf.h"