The names with multiple lowercase words run together are hard to read. We
started that way with very short names like rootprefix, but then same pattern
was applied to longer and longer names. Looking at the body of .pc files
available on my machine, many packages use underscores; let's do the same. Old
names are kept for compatiblity, so this is backwards compatible.
Let's not use atoi() if we can simply provide the project version as a number.
In C code, this is the numerical project version. In substitutions in other
files, this is just the bare substitution.
The "PACKAGE_" prefix is from autotools, and is strange. We call systemd a
"project", and "package" is something that distros build. Let's rename.
PACKAGE_URL is renamed to PROJECT_URL for the same reasons and for consistency.
(This leave PACKAGE_VERSION as the stringified define for C code.)
We defined both $(VERSION) and $(PACKAGE_VERSION) with the same contents.
$(PACKAGE_VERSION) is slightly more descriptive, so settle on that, and
drop the other define.
