lib/ukfile: Add backing buffer shared memory API

[andreittr]

Description of changes

This change expands the ukfile API to provide direct shared memory access to file backing buffers, useful for implementing memory mappings (mmap) or single-copy file-to-file transfers (sendfile).
In broad strokes, consumers would use it as such:

1. single-copy file-to-file (sendfile):

• lock source file
• iomem(BORROW)
• write into dest file the iovec received from above
• unlock source file

2. shared memory mapping (file mmaps):

• mmap() does iomem(ACQUIRE); this ensures
  (1) backing buffer is mapped by the file for the respective area and
  (2) addresses returned by iomem(RETRIEVE) are stable until RELEASE
• munmap() does iomem(RELEASE)
• sometime between mmap and paging in the memory, do iomem(RETRIEVE) to get specific memory ranges

3. memory gifting (i.e., make the file take some memory and use it as its own)

• call iomem(GIFT) with memory you own and want the file to take over
• like a reverse mmap: the file takes your memory as its backing buffer, rather than the other way around
• does not have a POSIX/Linux equivalent

A boilerplate no-op implementation is provided, and existing ukfiles are patched to use it.
No actual implementations or consumers of this API are included. Rather, the intent of this PR is to review the API and ensure it is properly documented in isolation.

Prerequisite checklist

• Read the contribution guidelines regarding submitting new changes to the project;
• Tested your changes against relevant architectures and platforms;
• Ran the checkpatch.uk on your commit series before opening this PR;
• Updated relevant documentation.

Base target

• Architecture(s): [e.g. x86_64 or N/A]
• Platform(s): [e.g. kvm, xen or N/A]
• Application(s): [e.g. app-python3 or N/A]

Additional configuration

N/A