fdtable: replace z_ prefix with zvfs_ for fdtable.h functions

For each of the fdtable.h functions listed below, convert the
z_ prefixed semi-private functions to use the zvfs_ prefix.

ZVFS stands for Zephyr Virtual File System and
is intended to be a common library used by the C library,
POSIX API, Networking, Filesystem, and other areas.

There are already a few functions in fdtable.h that use the
zvfs_ prefix, so this change is mostly about unifying them in
a way that uses a suitable prefix ("namespace") so that it can
be considered a public API.

- z_alloc_fd
- z_fdtable_call_ioctl
- z_finalize_fd
- z_finalize_typed_fd
- z_free_fd
- z_get_fd_obj
- z_get_fd_obj_and_vtable
- z_get_obj_lock_and_cond
- z_reserve_fd

Signed-off-by: Chris Friedt <cfriedt@tenstorrent.com>