Add POSIX fexecve.

I'm skeptical about the usefulness of this, but it's in POSIX, it's
in glibc (but not iOS), and it is used in some internal source (test
runners and container code).

Bug: N/A
Test: ran tests
Change-Id: I92c5398f2a679b21a33fba92bc8e67e3ae2eb76f
diff --git a/libc/include/unistd.h b/libc/include/unistd.h
index 9cfb918..fe98c10 100644
--- a/libc/include/unistd.h
+++ b/libc/include/unistd.h
@@ -98,6 +98,7 @@
 int execlp(const char* __file, const char* __arg0, ...) __attribute__((__sentinel__));
 int execle(const char* __path, const char* __arg0, ... /*,  char* const* __envp */)
     __attribute__((__sentinel__(1)));
+int fexecve(int __fd, char* const* __argv, char* const* __envp) __INTRODUCED_IN_FUTURE;
 
 int nice(int __incr);