Add capability to set the default scheme

Add a set_default_scheme(_: &CStr) function that determines how absolute paths are resolved.

Currently an absolute path that's not a legacy URL (X:Y) or a standard path with a scheme (/scheme/X/Y) were treated as if they were relative to /scheme/file. After calling set_default_scheme(X), they will now be relative to /scheme/X.

This will allow init programs to have an actuall root filesystem using /scheme/initfs.

Created related MRs so that the changeset can be properly reviewed and tested:

• bootstrap!14
• escalated!4
• init!13
• cookbook!397

All of those need to be merged for the feature to work. bootstrap and escalated MRs must be merged after this is merged and their redox-rt dependencies are adjusted.