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

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.