Replace bam2fq numbers with existing constants

bam2fq:
-Replace flag constants written as numbers like 0x40 with #defines
 from htslib/sam.h like BAM_FREAD1.
Remove global variables from samtools view

view:
Removes the global variables and replaces them with a structure
containing the settings for a run.
Add ability to strip aux tags to samtools view

view:
-Add ability to strip aux tags from reads.
Merge pull request #109 from aux_tag_strip of github.com/mp15/samtools