BUG/MEDIUM: compression: the deflate algorithm must use global settings as well

Global compression settings (windowsize and memlevel) were only considered
for the gzip algorithm but not the deflate algorithm. Since a single allocator
is used for both algos, if gzip was first initialized the memory with parameters
smaller than default, then initializing deflate after with default settings
would result in overusing the small allocated areas.

To fix this, we make use of deflateInit2() for deflate_init() as well.

Thanks to Godbach for reporting this bug, introduced by in 1.5-dev13 by commit
8b52bb38. No backport is needed.