libc: implement `feof()`

[ycsin]

This was originally added as a ticket under POSIX, but feof() is not a POSIX function. It is from C89.

https://en.cppreference.com/w/c/io/feof

The expectation from the POSIX API is that this function is implemented as part of whatever C library is in use.

[DanTGL]

I would like to work on this.

Any advice?

[cfriedt]

@DanTGL - we probably need some ZVFS implementation of this (in fdtable.h) and a feof() wrapper in the common libc.