Basic implementation of more sensible string mutating functions for display format string creation. (`strncat_n()`, `strncpy_n()`.)

TODO: Better `dest` parameter bounds checking.

Fortune for naka's current commit: Future small blessing − 末小吉

src/display.c

@@ -11,7 +11,24 @@
 #define FLAGSET(f, flag) (( (f) & (flag) ) == (flag))
 #define DFLAGSET(f, flagname) FLAGSET(f, DISPF_SHOW_ ## flagname)
 
-//TODO: This probably won't work. Write wrappers for strncpy(), strncat() that return number of bytes written isntead of useless pointer that we already have.
+// Returns number of bytes written to `dest`. Otherwise same as `strncpy()`.
+static usize strncpy_n(char*pOUT dest, const char* restrict src, usize n)
+{
+	usize i;
+	for(i=0;i<n;i++)
+		if(! (*dest++ = *src++)) return i;
+	dest[i] = 0;
+	return i;
+}
+
+// Returns number of bytes written to (dest+strlen(dest)). Otherwise same as `strncat()`.
+static inline usize strncat_n(char*pOUT dest, const char* restrict src, usize n)
+{
+	usize dl = strlen(dest);
+	return strncpy_n(dest+dl, src, n);
+}
+
+//TODO: This works, but seems inherantly unsafe. Rewrite to use the above `strn*_n()` functions for better length tracking.
 static usize _display_get_fmt(dispflags_t flags, bool matched, usize _len, char str[pOUT _len])
 {
 	register usize len = _len;