[ifmgmt] Avoid relying on global variable within ifcommon_exec()

[ifmgmt] Avoid relying on global variable within ifcommon_exec()

The getopt API defines optind as a global variable.  When used by the
"autoboot" command, the payload function passed to ifcommon_exec() may
result in a new iPXE script being executed; the commands therein would
then overwrite the value of optind.  On returning, ifcommon_exec()
would continue processing the list of interfaces from an undefined
point.

Fix by using a local variable to hold the index within the list of
interfaces.

Reported-by: Robin Smidsrød <robin@smidsrod.no>
Signed-off-by: Michael Brown <mcb30@ipxe.org>