build: Fixes for path handling

This MR is split into six commits.

Three are hopefully uncontroversial fixes:

• b74a512e build: Use join_paths for LOCALEDIR and public-api install_dir
  These were missed in dc46a4d9.
• b3d9a5d7 build: Ensure SYSCONFDIR is absolute
  Much like libexecdir, sysconfdir is relative if and only if contained within the prefix.
• 709135ad build: Fix a double space (whitespace)

Three refactor the use of paths:

• cc6cd9d5 build: Rename abs_header_dir to include_subdir
  This is a better name as it's not absolute but a subdirectory of the includedir, relative to the prefix. Also rename the identical install_dir in public-api.
• c5b5636f build: Make dbus_interface_dir absolute
  Removes the need for a separate abs_dbus_interface_dir.
• 7f1b4031 build: Gather common path variables in root meson.build
  This eliminates a lot of get_option and some join_paths.