[settings] Use a generic setting's own type as its default type

[settings] Use a generic setting's own type as its default type

When fetching a named setting using a name that does not explicitly
specify a type, default to using the type stored when the setting was
created, rather than always defaulting to "string".  This allows the
behaviour of user-defined settings to match the behaviour of
predefined settings (which have a sensible default type).

For example:

  set server:ipv4 192.168.0.1
  echo ${server}

will now print "192.168.0.1", rather than trying to print out the raw
IPv4 address bytes as a string.

The downside of this change is that existing tricks for printing
special characters within scripts may require (backwards-compatible)
modification.  For example, the "clear screen" sequence:

  set esc:hex 1b
  set cls ${esc}[2J
  echo ${cls}

will now have to become

  set esc:hex 1b
  set cls ${esc:string}[2J  # Must now explicitly specify ":string"
  echo ${cls}

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