Clean up the read routines.

The block read routines don't need to return a "bytes read" amount any
more.

Have pcapng_read_block() just return an indication:

	PCAPNG_BLOCK_OK - the read succeeded;

	PCAPNG_BLOCK_NOT_SHB - the read failed in a fashion that
	    indicates that we might just not be reading a pcap-ng file;

	PCAPNG_BLOCK_ERROR - the read failed in some other fashion
	    (i.e., we already have concluded that the file is a pcap-ng
	    file, or we got an I/O error).

In the cases where it needs to know whether it's reading the first block
for an open, have it check the shb_read flag, rather than being passed a
separate Boolean argument.

This means that pcapng_read_section_header_block() should return such an
indication as well.

Make the other block-reading routines return a Boolean success/failure
indication.

Change-Id: Id371457018a008ece9058d6042da44d631e51889
Reviewed-on: https://code.wireshark.org/review/4710
Reviewed-by: Guy Harris <guy@alum.mit.edu>