lib/posix-fdio: Add bincompat support for RWF_*

[andreittr]

Description of changes

This change adds values for RWF_* flags in posix-fdio, allowing it to interpret their meaning even without support from our (no)libc. This enhances binary compatibility.

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

[Mihnea0Firoiu]

All good.
Reviewed-by: Mihnea Firoiu mihneafiroiu0@gmail.com

[robertZamfir601]

Hello! The PR looks good, the functions are a lot clearer this way. I have two questions/observations though:

1. For RWF_NOWAIT, previously the flag was ignored, but the user could still read. Now we will return an error that might break some apps that worked previously. Could this be a problem?
2. Should we use uk_pr_warn_once for RWF_SYNC, RWF_DYNC and RWF_APPEND also?

Everything else looks good.