Add properly paired to stats and correct definition in flagstat

stats: Properly paired flag counting has been added as a summary
number.

flagstat: ensure that read with properly paired flag set is
mapped as per SAM spec.
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.
Add some more comments to tview

tview: add some additional comments to explain what we're doing.
Reenable tview and port to htslib

tview: reenable the tview command by reverting removal of components essential to
tview.  Also port tview over to using the htslib header files.  Several new files
are created to house existing bits of code which were part of one huge lump but
needed to be split out when that lump was deleted.
Restore sample selection to tview and add a unit test for the code doing it.

Allow a user to select which samples should appear in the tview by parsing
the header and determining which read groups belong to that sample.  Add a
unit test to exercise this bit of code.
Add a few comments to bam merging function
WIP on header merging
Add target id translation code and write pseudo code for building RG and PG translation tables
Convert PG and RG pseudo code to working code.
Add pseudocode for PG PP: translation.
Add -D_GNU_SOURCE for some of the newer POSIX functions
Further work on bam merge to fix header produced

Add code to append SQ tags to header
Fix asprintf format specifiers
Fix bugs picked up by unit testing.
Make trans_tbl_t a named struct and move aside variable names the same as it.
More work on merging @RG and @PG tags into header.
Unit tests for merge
add skeleton code for PP processing
NULLify some freed pointers.
Add tracing statement
Unit test trans_tbl_init add test 5 for PG tags
Fix memory leaks and finalise @PG PP handling
Test @PG merging and PP handling