Remove unneeded checks for prelen.

In order to determine the type of a compressed file, we have to read
in the first four bytes which may also be important for decompression
purposes, to do that we would pass the buffer that we have already
read in, along with the size of it.

Rename header1 to fourbytes to make that explicit, and remove all
checks for prelen.

Reported by: cem
Reviewed by: cem
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D24034