Added basic El Torito ISO image boot capability

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Make expand_buffer() a non-inline function, so that we can add debug
messages.

Expandable buffers now don't *always* round up to the next power of two.
This allows e.g. loading a 137MB ISO image on a machine with only 256MB
RAM...

around which still hold pointers to variables belonging to our parent...

correct prototypes for srandom(), rand() and srand().

is now gPXE-specific.  (The standard malloc() et al have been in stdlib.h
for a while).

Add free memory counter.

full-length PKB allocation.

protocols by URI scheme.

reflect the fact that they allocate and deallocate user memory (i.e.
things reached through a userptr_t).

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.

handle something as convoluted as

  ftp://joe:secret@insecure.org:8081/hidden/path/to?what=is#this

This bug found by setting read and write watchpoints at NULL in bochs.

do it.

Add IMAGE_PXE to use the new image format framework.  "kernel pxelinux.0"
now works.

that switches from line-oriented to byte-oriented partway through, such as
HTTP.

took out the contract to eventually call efree() when it called fetch().

Maintain the most recently loaded image at the start of the list, so that
imgautoselect() will pick it.

Add scripts as an image format (since it's trivial to do).

Avoid returning errors for comments and empty lines.

returns the allocated buffer.