build: Make the directory for helper executables consistent with Autotools

The Autotools build system has been using /usr/lib/polkit-1 for several releases, even on distributions where the library directory is /usr/lib64 or /usr/lib/x86_64-linux-gnu, so it makes sense for Meson to do the same. This lets 32- and 64-bit polkit agents share a single helper executable.

This might be superseded by !63 (closed), which requests going back to using the libexecdir for these (like polkit 0.105 did), which would also make sense; but until that's decided, let's at least be consistent between our two build systems.

Every time we change this, all programs that have already loaded libpolkit-agent into their address space need to be restarted, unless distributions provide compatibility symlinks.