Implement vmalloc to allocate memory from virtual heap space (ffs)

[salekinsirajus]

This function allocates the desired amount of memory (in bytes) off a process’s virtual heap space, and returns SYSERR if the allocation fails.

char* vmalloc (uint32 nbytes)

Virtual heap space should be allocated using the first-fit policy from lower to higher addresses. Note that this can cause external fragmentation of the virtual space.