Makefile.am: Don't race to build gdbus-codegen output

Prevent causes gdbus-codegen to run twice, once for the header and
once for the source code. Both invocations would generate both
files.
Makefile.am: Rework how we use GLIB_VERSION_MAX_ALLOWED

GLIB_VERSION_MAX_ALLOWED unless us to quickly see if we've used
any GLib APIs that are not supported by our target GLib version.

However gdbus-codegen is really broken in this regard and use
of GLIB_VERSION_MAX_ALLOWED where the version is less than 2.38
generates code which crashes. See this bug:

https://bugzilla.gnome.org/show_bug.cgi?id=710133

So we post-process the gdbus-codegen generated files to remove
the GLIB_VERSION_MAX_ALLOWED define when compiling those files.
Makefile.am: Cleaner use of CPPFLAGS and CFLAGS

Use AM_CPPFLAGS for our project global settings and individual
xxx_CFLAGS variables for the various components.
Makefile.am: Remove unused WARN_CFLAGS variable usage
Makefile.am: Put agent in its own directory and cleanup

Since we're going to be putting more into the agent, move
it to its own source directory.

Moved some common code into src/cockpit to be used
by both cockpit-agent and cockpit-ws.

While doing this, I did a bit of housekeeping in the
make files. This helps with code coverage by not building
lots of stuff multiple times.

Sorry that there's a bit mixed together here, but due to
time constraints I haven't split it all out into
separate patches.