In order to display syscall arguments in strace logging, we sometimes need to know the values of multiple syscall arguments. For example to display the byte buffer for write(), we need to know both const void *buf and size_t count. Shadow previously would just read bytes until a nul or a memory access error, but this wasn't correct since the buffer is often not nul-terminated.

To print a byte buffer like the one in write(), instead of using *const libc::c_char in the log_syscall() macro, we now have two types SyscallStringArg and SyscallBufferArg<const LEN_INDEX: usize> depending on whether the syscall takes a buffer (as in write()) or a nul-terminated string (as in open()). The constant generic in SyscallBufferArg allows us to specify the index of the count argument. This is a little weird, but allows us to continue using rust's type system for specifying how to display syscall arguments.

In the future we can use the same method to display other types of syscall arguments, like socket addresses or socket/ioctl options.