Fix various compiler warnings

[smcv]

Includes #32, which is the only compiler warning that seemed like it indicated a genuine bug; please review that one first.

• 32

• dbus-proxy: Treat GPtrArray index as unsigned

  This squashes some -Wsign-compare warnings.

• buffer_read: Be more careful with signedness

• flatpak-proxy: Silence compiler warnings about invalid casts

  GSource and -Wcast-function-type don't really get on, but G_SOURCE_FUNC has the appropriate magic to stop the compiler warning about this cast.

• flatpak-proxy: Move a case from an if() into a switch()

  Instead of special-casing EXPECTED_REPLY_NONE to be a separate if() block, treating it as part of the switch() lets us use -Wswitch-enum to warn if we have missed one.

• flatpak-proxy: Handle every case in a switch

  This lets us use -Wswitch-enum to detect when we've missed one.

• build: Add some more compiler warnings

  Taken from ostree, via bubblewrap.

• build: Add switch warnings

[smcv]

Updated to rebase on #32, and actually add the warnings to our build system now that #31 is in.