Move from read_gz to the pipe()+fork() method.

open_transformer(), common code for pipe+fork.
Function pointer for read() no longer needed.
Allow inflate to be initialised with a specified buffer size to avoid
over-reading.
Reset static variables in inflate_get_next_window to fix a bug where
only the first file in a .zip would be be extracted.

archival/libunarchive/Makefile.in

@@ -49,6 +49,7 @@ LIBUNARCHIVE-y:= \
 \
 	data_align.o \
 	find_list_entry.o \
+	open_transformer.o \
 	init_handle.o
 
 GUNZIP_FILES:= check_header_gzip.o unzip.o