getopt: save and restore context ...

execute_command is the single point where commands are executed and thus a new
getopt context is needed. currently we call getopt_reset here to reset the
context. This breaks though when a command tries to run a command itself by
calling execute_command or run_command.  In this case we have to store the
context and restore it afterwards. The same is necessary in builtin_getopt.
Currently noone does this so this one shouldn't fix a bug, but merely allows
us to do such things later.

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

svn_rev_282 ...

add getopt(3) implementation