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/realpath.c

@@ -145,7 +145,7 @@ restart:
 			l++;
 		}
 		if (q+l >= PATH_MAX) goto toolong;
-		memcpy(output+q, stack+p, l);
+		if (l) memcpy(output+q, stack+p, l);
 		output[q+l] = 0;
 		p += l;
 
@@ -223,7 +223,7 @@ skip_readlink:
 		if (q-p && !IsSlash(stack[l-1])) stack[l++] = '/';
 		if (l + (q-p) + 1 >= PATH_MAX) goto toolong;
 		memmove(output + l, output + p, q - p + 1);
-		memcpy(output, stack, l);
+		if (l) memcpy(output, stack, l);
 		q = l + q-p;
 	}