Bug fixes for ctx-sort and ctx-correct

BUG fixes:

1. Fix bug in ctx_sort.c for k > 31. We were not sorting by words after the first
   word in BinaryKmers.

2. ctx_sort.c should take the -f,--force command line option

3. correct_alignment.c was not setting aln->seq_gaps boolean properly. This was
   causing Ns in sequence to not trigger aln->seq_gaps to be true.

4. Try to get the username with getpwuid_r() if getlogin_r() fails

Improvements:

1. correct_reads.c: report number of gaps + num gaps filled

2. simplify db_alignment_print(aln) code - cleaner output for debugging