Change the PXE return code for EWOULDBLOCK from PXENV_STATUS_FAILURE
to PXENV_STATUS_TFTP_OPEN.  This code is only used by the FILE_READ
PXEXT call, and is necessary to distinguish "error" from "no data" in
that call.

(The only other nonblocking call is UDP_READ, where the caller doesn't
care about the distinction, however, gPXE doesn't use EWOULDBLOCK
internally to represent this condition in that code.)

posix_io.c and this PXE error makes more sense in this context.

will enable us to cascade async operations, which is necessary in order to
properly support DNS.  (For example, an HTTP request may have to redirect
to a new location and will have to perform a new DNS lookup, so we can't
just rely on doing the name lookup at the time of parsing the initial
URL).

Anything other than HTTP is probably broken right now; I'll fix the others
up asap.

as allocated by malloc()) versus all other errors, in particular
ENOBUFS (representing a lack of external buffer space, e.g. in a
device's TX queue or a user buffer).

is a DHCP option" data structures.

We need this code in order to be able to return a DHCP packet to a PXE NBP
which reflects options from our multiple sources (e.g. NVS and DHCP
server).  This is expensive, but necessary.  Having paid this cost, we may
as well try to use the same code to generate our DHCP request packets,
since the process is similar.

of them in one go.

EBADIMG has been replaced by ENOEXEC, and EIMGRET by ECANCELED.

I want to get to the point where any header in include/ reflects a
standard user-level header (e.g. a POSIX header), while everything that's
specific to gPXE lives in include/gpxe/.  Headers that reflect a Linux
header (e.g. if_ether.h) should also be in include/gpxe/, with the same
name as the Linux header and, preferably, the same names used for the
definitions.

us to return proper PXE status codes, while simultaneously allowing for
more consistent error reporting (complete with verbose error messages as a
build-time option).