Make more fixes and improvements

- Remove PAGESIZE constant
- Fix realloc() documentation
- Fix ttyname_r() error reporting
- Make forking more reliable on Windows
- Make execvp() a few microseconds faster
- Make system() a few microseconds faster
- Tighten up the socket-related magic numbers
- Loosen restrictions on mmap() offset alignment
- Improve GetProgramExecutableName() with getenv("_")
- Use mkstemp() as basis for mktemp(), tmpfile(), tmpfd()
- Fix flakes in pthread_cancel_test, unix_test, fork_test
- Fix recently introduced futex stack overflow regression
- Let sockets be passed as stdio to subprocesses on Windows
- Improve security of bind() on Windows w/ SO_EXCLUSIVEADDRUSE

libc/intrin/describestat.c

@@ -79,7 +79,7 @@ const char *(DescribeStat)(char buf[N], int rc, const struct stat *st) {
     append(", .st_%s=%'lu", "rdev", st->st_rdev);
   }
 
-  if (st->st_blksize != PAGESIZE) {
+  if (st->st_blksize != 4096) {
     append(", .st_%s=%'lu", "blksize", st->st_blksize);
   }