[settings] Make "netX" settings block function as a symbolic link

Add a facility for settings blocks to act as symbolic links to other
settings blocks, and reimplement the "netX" virtual settings block
using this facility.

The primary advantage of this approach is that unscoped settings such
as ${mac} and ${filename} will now reflect the settings obtained from
the most recently opened network device: in most cases, this will mean
the settings obtained from the most recent DHCP attempt.  This should
improve conformance to the principle of least astonishment.

Signed-off-by: Michael Brown <mcb30@ipxe.org>
[cmdline] Accept "netX" in iPXE commands

Allow any iPXE command expecting a network device name to accept
"netX" as a synonym for "most recently opened network device".

Signed-off-by: Michael Brown <mcb30@ipxe.org>