can't be saved in compress form" are both equivalent to "this file file
format requires seeking when writing it". Change the "can compress"
Boolean in the file format table to "writing requires seeking", give all
the entries the proper value, and do the checks for attempting to write
a file format to a pipe or write it in compressed format to common code.
This means we don't need to pass the "can't seek" flag to the dump open
routines.
svn path=/trunk/; revision=36575
Jakub Zawadzki