Allow multiple ioset backends.

Allow multiple ioset backends.

configure.in: Add framework to check for ioset backends.

src/Makefile.am (noinst_DATA): Reorganize for easier patching.
  (checkversion): Avoid spurious error message if $(GNU_ARCH) is missing.
  (EXTRA_srvx_SOURCES): Reorganize for easier patching; add ioset-select.c.
  (srvx_SOURCES): Add ioset-impl.h.

src/ioset.c (ioset.h): #include "ioset-impl.h" instead.
  (clock_skew): Make non-static for backends.
  (engine): New variable.
  (active_fd): New variable (kind of kludgy).
  (io_engine_*): New variables, only one exists so far.
  (ioset_init): New function.
  (ioset_cleanup): Call engine's cleanup function.
  (ioset_add): Call engine's add function.
  (ioset_listen): New function.
  (ioset_connect): Call engine's update functions at appropriate points.
  (ioset_try_write): Call engine's update function on success.
  (ioset_close): Change signature; check against active_fd; call engine's
    remove function.
  (ioset_accept): New function.
  (ioset_buffered_read): Update fd->state instead of fd->eof and call
    engine's update function as appropriate.  Use active_fd.
  (ioset_line_read): Check fd->state instead of fd->eof.
  (debug_fdsets): Move to ioset-select.c.
  (ioset_events): New function.
  (ioset_run): Move most of the logic into ioset-select loop function;
    call it.  Rename "select_timeout" to "timeout" to match.
  (ioset_write): Call engine->update function.
  (ioset_printf): New function.

src/ioset.h (common.h): #include this header to get PRINTF_LIKE macro.
  (struct io_fd): Replace 'connected' and 'eof' fields with 'state' field.
  (ioset_init): Declare new function.
  (ioset_listen): Declare new function.
  (ioset_printf): Declare new function.
  (ioset_close): Update signature.

src/main.c (main): Call ioset_init().

src/mod-sockcheck.c (sockcheck_free_client): Can unconditionally call
    ioset_close() now.
  (expand_var): Always use C99 type names.
  (sockcheck_begin_test): Can unconditionally call ioset_close() now.
  (sockcheck_read_conf): Only warn about unknown host if the user set
    one; it is silly to arn about unknown host `(null)'.

src/proto-common.c (socket_destroyed): Check fd->state rather than
    fd->eof.
  (close_socket): Update signature for ioset_close().
git-archimport-id: srvx@srvx.net--2006/srvx--devo--1.3--patch-38