Zstandard v1.2.0

Major features :

• Multithreading is enabled by default in the cli. Use -T# to select nb of thread. To disable multithreading, build target zstd-nomt or compile with HAVE_THREAD=0.
• New dictionary builder named "cover" with improved quality (produces better compression ratio), by @terrelln. Legacy dictionary builder remains available, using --train-legacy command.

Other changes :
cli : new : command -T0 means "detect and use nb of cores", by @iburinoc
cli : new : zstdmt symlink hardwired to zstd -T0
cli : new : command --threads=# (#671)
cli : new : commands --train-cover and --train-legacy, to select dictionary algorithm and parameters
cli : experimental targets zstd4 and xzstd4, supporting lz4 format, by @iburinoc
cli : fix : does not output compressed data on console
cli : fix : ignore symbolic links unless --force specified,
API : breaking change : ZSTD_createCDict_advanced() uses compressionParameters as argument
API : added : prototypes ZSTD_*_usingCDict_advanced(), for direct control over frameParameters.
API : improved: ZSTDMT_compressCCtx() reduced memory usage
API : fix : ZSTDMT_compressCCtx() now provides srcSize in header (#634)
API : fix : src size stored in frame header is controlled at end of frame
API : fix : enforced consistent rules for pledgedSrcSize==0 (#641)
API : fix : error code GENERIC replaced by dstSizeTooSmall when appropriate
build: improved cmake script, by @Majlen
build: enabled Multi-threading support for *BSD, by @bapt
tools: updated paramgrill. Command -O# provides best parameters for sample and speed target.
new : contrib/linux-kernel version, by @terrelln