Make improvements

- Introduce ualarm() function
- Make rename() report EISEMPTY on Windows
- Always raise EINVAL upon open(O_RDONLY|O_TRUNC)
- Add macro so ./configure will detect SOCK_CLOEXEC
- Fix O_TRUNC without O_CREAT not working on Windows
- Let fcntl(F_SETFL) change O_APPEND status on Windows
- Make sure pwrite() / pread() report ESPIPE on sockets
- Raise ESPIPE on Windows when pwrite() is used on pipe
- Properly compute O_APPEND CreateFile() flags on Windows
- Don't require O_DIRECTORY to open directories on Windows
- Fix more instances of Windows reporting EISDIR and ENOTDIR
- Normalize EFTYPE and EMLINK to ELOOP on NetBSD and FreeBSD
- Make unlink() / rmdir() work on read-only files on Windows
- Validate UTF-8 on Windows paths to fix bug with overlong NUL
- Always print signal name to stderr when crashing due to SIG_DFL
- Fix Windows bug where denormalized paths >260 chars didn't work
- Block signals on BSDs when thread exits before trashing its own stack

libc/sock/sendmsg.c

@@ -76,7 +76,7 @@ ssize_t sendmsg(int fd, const struct msghdr *msg, int flags) {
     } else if (__isfdkind(fd, kFdSocket)) {
       rc = sys_sendto_nt(fd, msg->msg_iov, msg->msg_iovlen, flags,
                          msg->msg_name, msg->msg_namelen);
-    } else if (__isfdkind(fd, kFdFile)) {
+    } else if (__isfdkind(fd, kFdFile)) {  // e.g. socketpair
       rc = sys_write_nt(fd, msg->msg_iov, msg->msg_iovlen, -1);
     } else {
       rc = enotsock();