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/calls/execvpe.c

@@ -27,7 +27,9 @@
 /**
  * Executes program, with path environment search.
  *
- * The current process is replaced with the executed one.
+ * This function is a wrapper of the execve() system call that does path
+ * resolution. The `PATH` environment variable is taken from your global
+ * `environ` rather than the `envp` argument.
  *
  * @param prog is the program to launch
  * @param argv is [file,argv₁..argvₙ₋₁,NULL]
@@ -47,6 +49,10 @@ int execvpe(const char *prog, char *const argv[], char *const *envp) {
     return efault();
   }
 
+  if (strchr(prog, '/')) {
+    return execve(prog, argv, envp);
+  }
+
   // resolve path of executable
   if (!(exe = commandv(prog, pathbuf, sizeof(pathbuf)))) {
     return -1;