Bug 4875 pt1: GCC-8 compile errors with -O3 optimization (#287)

Bug 4875 pt1: GCC-8 compile errors with -O3 optimization (#287)

 Use xstrncpy instead of strncat for String appending

Our xstrncpy() is safer, not assuming the existing char*
is nul-terminated and accounting explicitly for the
nul-terminator byte.

GCC-8 -O3 optimizations were exposing a strncat() output
truncation of the terminator when insufficient space was
available in the String buffer.