Make improvements

- This change fixes a bug that allowed unbuffered printf() output (to
  streams like stderr) to be truncated. This regression was introduced
  some time between now and the last release.

- POSIX specifies all functions as thread safe by default. This change
  works towards cleaning up our use of the @threadsafe / @threadunsafe
  documentation annotations to reflect that. The goal is (1) to use
  @threadunsafe to document functions which POSIX say needn't be thread
  safe, and (2) use @threadsafe to document functions that we chose to
  implement as thread safe even though POSIX didn't mandate it.

- Tidy up the clock_gettime() implementation. We're now trying out a
  cleaner approach to system call support that aims to maintain the
  Linux errno convention as long as possible. This also fixes bugs that
  existed previously, where the vDSO errno wasn't being translated
  properly. The gettimeofday() system call is now a wrapper for
  clock_gettime(), which reduces bloat in apps that use both.

- The recently-introduced improvements to the execute bit on Windows has
  had bugs fixed. access(X_OK) on a directory on Windows now succeeds.
  fstat() will now perform the MZ/#! ReadFile() operation correctly.

- Windows.h is no longer included in libc/isystem/, because it confused
  PCRE's build system into thinking Cosmopolitan is a WIN32 platform.
  Cosmo's Windows.h polyfill was never even really that good, since it
  only defines a subset of the subset of WIN32 APIs that Cosmo defines.

- The setlongerjmp() / longerjmp() APIs are removed. While they're nice
  APIs that are superior to the standardized setjmp / longjmp functions,
  they weren't superior enough to not be dead code in the monorepo. If
  you use these APIs, please file an issue and they'll be restored.

- The .com appending magic has now been removed from APE Loader.

libc/stdio/dirstream.c

@@ -513,6 +513,7 @@ static struct dirent *readdir_impl(DIR *dir) {
  * @param dir is the object opendir() or fdopendir() returned
  * @return next entry or NULL on end or error, which can be
  *     differentiated by setting errno to 0 beforehand
+ * @threadunsafe
  */
 struct dirent *readdir(DIR *dir) {
   struct dirent *e;
@@ -535,7 +536,6 @@ struct dirent *readdir(DIR *dir) {
  * @param result will receive `output` pointer, or null on eof
  * @return 0 on success, or errno on error
  * @returnserrno
- * @threadsafe
  */
 errno_t readdir_r(DIR *dir, struct dirent *output, struct dirent **result) {
   int err, olderr;
@@ -587,7 +587,6 @@ int closedir(DIR *dir) {
 
 /**
  * Returns offset into directory data.
- * @threadsafe
  */
 long telldir(DIR *dir) {
   long rc;
@@ -599,7 +598,6 @@ long telldir(DIR *dir) {
 
 /**
  * Returns file descriptor associated with DIR object.
- * @threadsafe
  */
 int dirfd(DIR *dir) {
   return dir->fd;
@@ -607,7 +605,6 @@ int dirfd(DIR *dir) {
 
 /**
  * Seeks to beginning of directory stream.
- * @threadsafe
  */
 void rewinddir(DIR *dir) {
   lockdir(dir);
@@ -634,7 +631,6 @@ void rewinddir(DIR *dir) {
 
 /**
  * Seeks in directory stream.
- * @threadsafe
  */
 void seekdir(DIR *dir, long tell) {
   lockdir(dir);