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/gunzip.c

@@ -179,11 +179,10 @@ extern int gunzip_main(int argc, char **argv)
 #endif
 				if (magic2 == 0x8b) {
 					check_header_gzip(src_fd);
-					status = inflate(src_fd, dst_fd);
+					status = inflate_gunzip(src_fd, dst_fd);
 					if (status != 0) {
 						bb_error_msg_and_die("Error inflating");
 					}
-					check_trailer_gzip(src_fd);
 				} else {
 					bb_error_msg_and_die("Invalid magic");
 				}