Work around system restrictions regarding exec

Work around system restrictions regarding exec

* doc/emacs/android.texi (Android Environment): Document
`android-use-exec-loader'.
* exec/exec1.c (main): Set program group of child process.
* src/android.c (android_rewrite_spawn_argv): New function.
* src/android.h: Update prototypes.
* src/androidfns.c (syms_of_androidfns): New variable
`android_use_exec_loader'.
* src/callproc.c (emacs_spawn): Rewrite the argument vector to
use exec1 if necessary.