changed name frameParams into frameHeader

changed name frameParams into frameHeader

ZSTD_frameParams => ZSTD_frameHeader
ZSTD_getFrameParams() -> ZSTD_getFrameHeader()

The new naming is more distinctive from ZSTD_frameParameters,
which is used during compression.

ZSTD_frameHeader is clearer in its intention to described frame header content.
It also implies we are decoding a ZSTD frame, hence we are at decoding stage.