lib/posix-fd: Move heap-alloc ofiles out of fdtab

lib/posix-fd: Move heap-alloc ofiles out of fdtab

This change moves the responsibility for allocating, managing the
lifetime of, and freeing heap-allocated open file descriptions out of
individual fdtab instances and into the main posix-fd API.
This allows libraries to create kernel-wide dynamic open file
descriptions independent of an fdtab.

Signed-off-by: Andrei Tatar <andrei@unikraft.io>
Approved-by: Sergiu Moga <sergiu@unikraft.io>
Reviewed-by: Sergiu Moga <sergiu@unikraft.io>
GitHub-Closes: #1602