BIO_s_mem()

Uses MEM_BUF *; the pointer is actually kept in BIO->ptr.

This source/sink is bi-directional and should not return errors other than
0 bytes left.  Any errors are due to malloc() failures.

Normal setup is just

bio=BIO_new(BIO_s_men());

BIO_gets does a fgets.
BIO_puts does a BIO_put().

/* woops, the folowing need to be fixed :-) */

BIO_CTRL_RESET does a fseek() back to the start of the file.

BIO_CTRL_EOF does a feof().

BIO_CTRL_INFO does a ftell().

BIO_CTRL_SET assigned the FILE * and close flag.

BIO_CTRL_GET returns the FILE * via the ptr parameter (FILE **).

BIO_CTRL_GET_CLOSE/BIO_CTRL_SET_CLOSE returns and sets the close on
free flag.

BIO_CTRL_PUSH returns 0, does nothing.
BIO_CTRL_POP returns 0, does nothing.

BIO_CTRL_PENDING unimplemented, returns 0.

BIO_CTRL_FLUSH fflush()es the FILE *.

BIO_CTRL_SHOULD_RETRY/BIO_CTRL_RETRY_TYPE unimplemented, returns 0.

BIO_CTRL_SET_FILENAME is a special case.  The file name is opened and
used.  This is a special case.

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