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/execlp.c

@@ -42,11 +42,6 @@ int execlp(const char *prog, const char *arg, ... /*, NULL*/) {
   va_list va, vb;
   char pathbuf[PATH_MAX];
 
-  // resolve path of executable
-  if (!(exe = commandv(prog, pathbuf, sizeof(pathbuf)))) {
-    return -1;
-  }
-
   // turn varargs into array
   va_copy(vb, va);
   va_start(va, arg);
@@ -60,6 +55,15 @@ int execlp(const char *prog, const char *arg, ... /*, NULL*/) {
   }
   va_end(vb);
 
+  if (strchr(prog, '/')) {
+    return execv(prog, argv);
+  }
+
+  // resolve path of executable
+  if (!(exe = commandv(prog, pathbuf, sizeof(pathbuf)))) {
+    return -1;
+  }
+
   // change argv[0] to resolved path if it's ambiguous
   // otherwise the program won't have much luck finding itself
   if (argv[0] && *prog != '/' && *exe == '/' && !strcmp(prog, argv[0])) {