Objects typically call xfer_close() as part of their response to a
close() message.  If the initiating object has already nullified the
xfer interface then this isn't a problem, but it can lead to
unexpected behaviour when the initiating object is aiming to reuse the
connection and so does not nullify the interface.

Fix by always temporarily nullifying the interface during xfer_close()
(as was already being done by xfer_vreopen() in order to work around
this specific problem).

Reported-by: infernix <infernix@infernix.net>
Tested-by: infernix <infernix@infernix.net>

Add FILE_LICENCE declarations to almost all files that make up the
various standard builds of gPXE.

When handling a redirection event, we need to close the existing
connection before opening the new connection.

Intel's C compiler (icc) chokes on the zero-length arrays that we
currently use as part of the mechanism for accessing linker table
entries.  Abstract away the zero-length arrays, to make a port to icc
easier.

Introduce macros such as for_each_table_entry() to simplify the common
case of iterating over all entries in a linker table.

Represent table names as #defined string constants rather than
unquoted literals; this avoids visual confusion between table names
and C variable or type names, and also allows us to force a
compilation error in the event of incorrect table names.

current working URI.

from the normal POSIX-style open()

Dehyphenate "data-transfer".