rtl8139: enable bus mastering if needed

This resolves #2.

Change-Id: Ie6736f0d5a713025f09c18de7042ba44fbb1b2f8
dhcpd: don't crash so easily

The jump-to-start hack is not resetting global variables, resulting in
a crash after several such restarts cause an overflow in the network
table.  This patch fixes that particular issue, but more similar
issues (in particular memory leaks) are bound to come up.  As such this
is a stopgap measure until we can get rid of the old dhcpd altogether.

This resolves the additional issue reported in #2.

Change-Id: Ic4cd80eae520cf2b97e893bac63b3ab1ecfea6d8