Improve standard compliance for memset_s() and abort_handler_s().

abort_handler_s() currently simply calls abort(), though the standard
specifies more: "Writes an implementation-defined message to stderr
which must include the string pointed to by msg and calls abort()."

memset_s() is missing error condition "n > smax", and does not invoke
the constraint handler after filling the buffer: "following errors are
detected at runtime and call the currently installed constraint
handler function after storing ch in every location of the destination
range [dest, dest+destsz) if dest and destsz are themselves valid",
one of the errors is "n > smax" itself.

Submitted by: Yuri Pankov <yuripv@gmx.com>
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D11991