* scmsigs.h, async.h: updated.

* scmsigs.h, async.h: updated.

* _scm.h: if HAVE_RESTARTS is defined then don't use a SYSCALL
loop.

* posix.c (scm_uname): interpret only negative values as an error.
Solaris normally returns a positive value.

* script.c (scm_compile_shell_switches): if we are not going into
an interactive repl, set scm_mask_ints to zero so that asyncs can
run.

* simpos.c (scm_system):  don't ignore/unignore signals around
the "system" call.

* posix.c (scm_open_pipe): don't ignore/unignore signals around
the "popen" call.

* init.c (scm_boot_guile_1): don't call scm_init_signals, it's
done in boot-9.scm instead.

* scmsigs.c, async.c: Major rewriting of signal handling code.
(scm_sigaction): new procedure.
(scm_sleep): don't wrap sleep in SCM_SYSCALL, it would mess up the
timing.
(scm_raise): return unspecified, throw error on failure.

* boot-9.scm: signal-handler, alarm-thunk: removed.
don't define ticks-interrupt etc.
top-repl: install signal handlers for SIGINT, SIGFPE, SIGSEGV, SIGBUS
during call to scm-style-repl.

* acconfig.h: mention HAVE_RESTARTS.
* configure.in: check for sigaction and restartable system calls.