Migrate ideas from bam2fq from htslib and document

Migrate the ideas for revision of bam2fq over from the htslib version.
Document bam2fq and ensure it is visible on samtools usage list.

Signed-off-by: Martin O. Pollard <mp15@sanger.ac.uk>
Miscellaneous code tweaks

Make bam2fq_usage()'s parameter directly understandable, undo gratuitous
C99-style declaration changes, reuse linebuf, use print_error_errno(),
fix switch, clear initial linebuf, free previous clearly.
Check for Usage output on stdout (ok) or stderr (expected failure)

Also fix "not implemented" typo.
Merge bam2fq improvements and documentation (PR #204)

This keeps the existing "add /1 or /2" default and -n option to suppress
it, rather than the pull request's -a option (which has been left out);
this seems to be the better information-preserving default.

(Fixes #168.)