Add copyright notices and licensing boilerplate

The file bcftools.1 is generated from this file, and it remains to find
a way to have copyright and licensing information generated into it.
Update license file

Rename to LICENSE, as the filename COPYING usually suggests GPL.
(Use git diff -M40% to see this commit as a rename.)

Use the less ambiguous term "MIT/Expat" license, as there are several
different licenses sometimes called the "MIT license".

Add note about year ranges, and reformat the last paragraph -- there
are no text changes.
Write out description and author in each source file header
Update copyright years to reflect when changes were made

Added the year 2012 to mcall.c's notice, as it contains code from
samtools-0.1.x/bcftools/prob1.c written by Petr Danecek in 2012.

Also write out description and author in each source file header.
Add previously-omitted copyright notices

ccall.c contains some code (ttest(), test16(), etc) from
samtools-0.1.x/bcftools/call1.c written by Heng Li while at Broad.

em.c, prob1.c, and prob1.h originate from similarly-named files in
samtools-0.1.x/bcftools (also samtools-0.1.x/bcftools/ld.c for em.c),
originally written by Heng Li at Broad in 2010 and modified thereafter
by him and by others at Sanger.

tabix.c was originally written by Heng Li at Broad in 2012, and modified
thereafter at Sanger.

Also write out description and author in each source file header.