AK+Kernel: Support snprintf

In contrast to sprintf, which might overflow the given buffer.

I feel bad about the code duplication, but that is a pre-existing issue.

AK/kstdio.h

@@ -37,6 +37,7 @@ int vdbgprintf(const char* fmt, va_list);
 int dbgprintf(const char* fmt, ...);
 ssize_t dbgputstr(const char*, ssize_t);
 int sprintf(char* buf, const char* fmt, ...);
+int snprintf(char* buffer, size_t, const char* fmt, ...);
 }
 #    endif
 #else