Use mmap and cache the view buffer for get_view

Use mmap and cache the view buffer for get_view

This patch uses mmap if it is available and works.  It also caches the
view buffer for get_view.

	* configure.ac: Add AC_FUNC_MMAP.
	* config.in: Regenerated.
	* configure: Likewise.
	* plugin.c: Include <sys/mman.h>.
	(MAP_FAILED): New.  Defined if not defined.
	(PROT_READ): Likewise.
	(MAP_PRIVATE): Likewise.
	(view_buffer_t): New.
	(plugin_input_file_t): Add view_buffer.
	(get_view): Try mmap and cache the view buffer.
	(plugin_maybe_claim): Initialize view_buffer.