common/iobuf.c: Combine iobuf_open, iobuf_create and iobuf_openrw.
* common/iobuf.c (do_open): New function, which is a generalization of iobuf_open, iobuf_Create, iobuf_openrw. (iobuf_open): Call do_open. (iobuf_create): Likewise. (iobuf_openrw): Likewise.
- Signed-off-by: Neal H. Walfield <neal@g10code.com>.