tools/nolibc/string: add strncpy() and strlcpy()

These are minimal variants. strncpy() always fills the destination for
<size> chars, while strlcpy() copies no more than <size> including the
zero and returns the source's length. The respective sizes on various
archs are:

  strncpy(): x86:0x1f mips:0x30 arm:0x20
  strlcpy(): x86:0x17 mips:0x34 arm:0x1a

Signed-off-by: Willy Tarreau <w@1wt.eu>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>