Fixed issues with buffering reads

This commit fixes the BUG caused by calling seq_guess_fastq_format multiple times
on the same stream.

1. sf->origread always points to one of:
* _seq_read_sam
* _seq_read_fast[aq]
* _seq_read_plain
* _seq_read_unknown

2. sf->read points to either sf->origread or _seq_read_pop

3. Fixed issues with read_t->next not being initialised

4. when reading sam/bam files read_t->origread is now set properly