When looking at how dlopen for various libs is implemented, I found that the
macros hide too much. I find it much easier to see what is going on if 'extern'
and '= NULL' are written explicitly. After all, we don't hide those for other
definitions, e.g. our style guide says that static variables should be
initialized with '= NULL'. With that change, it's much more obvious what is
a variable declaration and what is a variable initialization.
Dynamically load liblz4, libzstd and liblzma with dlopen().
This helps to reduce the size of the initrd image when these libraries
are not really needed.
Given we have two different types for the journal object flags and the
Compression enum, let's make the latter a regular non-sparse enum, and
thus remove some surprises. We have to convert anyway between the two,
and already do via COMPRESSION_FROM_OBJECT().
The compression helpers are used both in journal code and in coredump
code, and there's a good chance we'll use them later for other stuff.
Let's hence move them into src/basic/, to make them a proper internal
API we can use from everywhere where that's desirable. (pstore might be
a candidate, for example)
No real code changes, just some moving around, build system
rearrangements, and stripping of journal-def.h inclusion.
