[kernel,libc] Add _fmemfree system call and wrapper functions

[ghaerr]

Adds the ability to free memory allocated by fmemalloc. This will likely be required for large/huge memory allocations for the JWasm port in #2103. Supports ELKS and OWC compilers/libraries.

Usage:

#include <stdlib.h>
...
    char __far *p = fmemalloc(0x10000L);  // allocate 64K of main memory
...
    fmemfree(p); // free 64k memory

Updates fmemalloc man pages in sections 2 and 3.
Also adds _HShift global for OWC huge pointer support, required in pia.asm.