[settings] Clarify usage of the term "named setting"

[settings] Clarify usage of the term "named setting"

There are currently two conflicting usages of the term "named setting"
within iPXE: one refers to predefined settings (such as show up in the
"config" UI), the other refers to settings identified by a name (such
as "net0.dhcp/ip").

Split these usages into the term "predefined setting" and "named
setting" to avoid ambiguity.

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