zstd_decompress: use a helper function for context create

zstd_decompress: use a helper function for context create

Multiple ZSTD_createDCtx* functions call other (public)
ZSTD_createDCtx* functions, this makes it harder for humans
and compilers to throw out code that is not used.

This farms out the logic into a static function, if a program
only uses a single ZSTD_createDCtx variant, all others can be easily
dropped and the remaining implementation can be specialized.