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/calls/mkntpathat.c

@@ -29,9 +29,10 @@ int __mkntpathat(int dirfd, const char *path, int flags,
                  char16_t file[hasatleast PATH_MAX]) {
   char16_t dir[PATH_MAX];
   uint32_t dirlen, filelen;
+  if (!isutf8(path, -1)) return eilseq();  // thwart overlong nul in conversion
   if ((filelen = __mkntpath2(path, file, flags)) == -1) return -1;
   if (!filelen) return enoent();
-  if (file[0] != u'\\' && dirfd != AT_FDCWD) { /* ProTip: \\?\C:\foo */
+  if (file[0] != u'\\' && dirfd != AT_FDCWD) {  // ProTip: \\?\C:\foo
     if (!__isfdkind(dirfd, kFdFile)) return ebadf();
     dirlen = GetFinalPathNameByHandle(g_fds.p[dirfd].handle, dir, ARRAYLEN(dir),
                                       kNtFileNameNormalized | kNtVolumeNameDos);