build: untangle sysconfdir & localstatedir

build: untangle sysconfdir & localstatedir

`--sysconfdir` should be `/etc` and `--localstatedir` should be `/var`.
The package-specific subdirectory should be added by configure, not
given by the user, to match established behavior by other packages.

Note that `--bindir`, `--sbindir`, `--libdir` and `--libexecdir` have
different established/expected behavior due to distro specific
multi-arch support.  That's why these are left unchanged.

The reason this is getting fixed now is that we need to use
`--localstatedir` for its actual value to put things in `/var/lib`.  As
it is now, being overloaded for `/run`, the configured `/var` path
becomes inaccessible.

Signed-off-by: David Lamparter <equinox@opensourcerouting.org>