Update string.h, implement calloc() and realloc()

mem*() functions are now defined in klib/string.c.  klib/mem.c is used
for memory allocation functions, now including calloc() and realloc() in
addition to malloc() (the realloc() implementation is extremely
inefficient and should be avoided if at all possible).  The mem*()
declarations have also moved to <string.h> where they belong, and some
documentation has been added in klib/string.c.
Add stdlib.h, fix read/write signatures

Memory allocation routines are now declared in stdlib.h.  read() and
write() have standard UNIX signatures.
Purge telos/mem.h

Declarations moved to stdlib.h
Move things into unistd.h and klib/unistd.c

syssleep() is renamed sleep(), and its resolution is increased to
seconds (from milliseconds) to comply with standard UNIX sleep function.
sleep(), alarm() and getpid() definitions have been moved into
klib/unistd.c, with declarations in <unistd.h>.

Other minor changes:
    * <telos/filedes.h> incorporated into <unistd.h> and removed
    * <mem.h> moved to <kernel/mem.h> since it's really a kernel header
    * `size' parameter to kmalloc()/hmalloc() changed to unsigned int