Add a few missing __*_chk functions

When source code is compiled with -D_FORTIFY_SOURCE on Linux, various
functions are sometimes replaced by __*_chk variants (e.g., __strcpy_chk)
which can help avoid buffer overflows when the compiler knows the buffer's
size during compilation.

If we want to run source compiled on Linux with -D_FORTIFY_SOURCE (either
deliberately or unintentionally - see issue #111), we need to implement
these functions otherwise the program will crash because of a missing
symbol. We already implement a bunch of _chk functions, but we are
definitely missing some more.

This patch implements 6 more _chk functions which are needed to run
the "rogue" program (mentioned in issue #111) when compiled with
-D_FORTIFY_SOURCE=1.

Following the philosophy of our existing *_chk functions, we do not
aim for either ultimate performance or iron-clad security for our
implementation of these functions. If this becomes important, we
should revisit all our *_chk functions.

When compiled with -D_FORTIFY_SOURCE=2, rogue still doesn't work, but
not because of a missing symbol, but because it fails reading the
terminfo file for a yet unknown reason (a patch for that issue will
be sent separately).

Signed-off-by: Nadav Har'El <nyh@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>