'exit' used to do its job by returning value < 0. This is a sign
for hush that 'exit' is executed. This has problems:

- Often commands accidently return a negative value. This causes
  the shell to exit.
- execute_binfmt returns a negative value when it does not find
  a binary to execute. This again causes the shell to exit.
  Returning a negative error value seems to be the right thing
  to do, but catching this in the shell would mean that the exit
  command does not work anymore.
- if called without arguments exit is supposed to return the code
  of the last command. As a command exit has no access to this code.

This patch changes exit to be a builtin and also fixes the last return
code problem. While at it, update the help text.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>