Rewrite bed_read

Makes bed_read work line-by-line, and adds new BED parsing code that
hopefully better understands the format.  BED comments are explicitly
stripped.  Browser and track lines are ignored.  Columns can be separated
by multiple whitespace characters.  There is more, but not complete,
error checking.  The VCF-style two column format is still supported.
Includes big comment.
Adds check to ensure bed_read worked, and exits non-zero if not.
Stops samtools view from writing unmapped reads with -L (BED file) option

This affects reads with no RNAME, i.e. completely unmapped.
add -U FILE to output unselected reads for pipelin
Don't malloc the kstring_t; minor changes to the big comment
Merge BED file parsing improvements (PR #165)
Merge view -U option (PR #177), also adding it to the man page