vcf-merge: Reset -c before setting it

git-svn-id: https://svn.code.sf.net/p/vcftools/code@913 19d466ce-640d-45be-954f-fad4d9839663
vcf-merge: sum I16 and MQ0 too

git-svn-id: https://svn.code.sf.net/p/vcftools/code@914 19d466ce-640d-45be-954f-fad4d9839663
Merge remote-tracking branch 'upstream'