Rewrite glib-pacrunner to avoid libproxy
We shouldn't need libproxy to support proxy autoconfig. We can probably make this work by lifting code out of libproxy into glib-pacrunner. This will fix two problems:
- Currently libproxy is unsafe since it may link to mozjs or JSC versions incompatible with the application, but glib-pacrunner runs out-of-process and avoids this problem
- Currently glib-pacrunner is itself unsafe because it uses g_setenv() to configure libproxy