runuser: new applet

Add a cut down, Windows-specific implementation of `runuser` from
util-linux.

This allows elevated privileges to be dropped when running in an
SSH session.  It also works when using `su` or starting busybox-w32
'as administrator'.

There are complications:

- The method used to drop privileges leaves the access token in the
  TokenIsElevated state.  Detecting this is likely to be fragile.

- The unprivileged shell is started by CreateProcessAsUserA().  In
  older versions of Windows this has to be loaded dynamically.

Adds about 900 bytes.

(GitHub issue #240)

1 files changed, 2 insertions, 0 deletions

diff --git a/include/mingw.h b/include/mingw.h
index b5b2fe169..e49483307 100644
--- a/include/mingw.h
+++ b/include/mingw.h
@@ -590,3 +590,5 @@ int has_path(const char *file);
 int is_relative_path(const char *path);
 char *get_last_slash(const char *path);
 const char *applet_to_exe(const char *name);
+char *get_user_name(void);
+char *quote_arg(const char *arg);