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.

src/options.c

@@ -400,14 +400,27 @@ void _mi_error_message(int err, const char* fmt, ...) {
 // --------------------------------------------------------
 
 static void mi_strlcpy(char* dest, const char* src, size_t dest_size) {
-  dest[0] = 0;
-  strncpy(dest, src, dest_size - 1);
-  dest[dest_size - 1] = 0;
+  if (dest_size == 0)
+    return;
+
+  // Copy until end of 'src' or dest is (almost) full
+  while (*src && (dest_size > 1)) {
+    *dest++ = *src++;
+    --dest_size;
+  }
+  // Null-terminate dest
+  *dest = 0;
 }
 
 static void mi_strlcat(char* dest, const char* src, size_t dest_size) {
-  strncat(dest, src, dest_size - 1);
-  dest[dest_size - 1] = 0;
+  // Skip existing data in 'dest'
+  while (*dest && (dest_size > 1)) {
+    ++dest;
+    --dest_size;
+  }
+
+  // Concatenate src
+  mi_strlcpy(dest, src, dest_size);
 }
 
 #ifdef MI_NO_GETENV