Replace mi_strlcpy() and mi_strlcat() with versions written from scratch
Replace mi_strlcpy() and mi_strlcat() with versions written from scratch

They used strncpy() and strncat(), which behave almost, but not
quite like the ...l... functions.
Since these functions are not standard, and not all OSes have
comparable functions available, just add a implementations.

Addresses first issue raised in microsoft/mimalloc#502.

microsoft/mimalloc,microsoft/mimalloc