Man page formatting fixes

Use hyphens to introduce options rather than en dashes
(addresses samtools/www.htslib.org#5 for samtools man page).
Write failure messages to stderr

No difference by default, but enables highlighting the failures or
capturing them by redirecting stderr.
Maintain HEAP_EMPTY invariant when an input is empty

Ensure h->b is NULL when we hit EOF on the first read, just as it is
when we hit EOF later in the file.  Fixes #309 and also prevents a
memory leak for an input file with no records.

Also use bam_init1()/bam_destroy1() instead of creating and destroying
bam1_t structs by hand.
Reflect changes in htslib API