Tidy up code

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

of blocking on asynchronous operations, when that isn't an important
aspect of the code.

it as a fallback network device rather than an explicitly forced network
device.

Clear routing table entries before re-attempting DHCP.

exception rather than the rule; we may as well keep things clean for other
protocols.

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

return a static buffer.

tcpip_tx().  This avoids the irritating wait when you accidentally type
"kernel pxelinux.0" before bringing up the network interface.

Add ENETUNREACH to strerror()'s list.

retrying".

doesn't want to go to the hassle of processing a full memory map.

initrds.

Parse command line for "vga=" and "mem=" parameters

Add a path for isolinux.bin that is correct on my system; hopefully it
will work on others too.

Move pxe_boot() code to pxe_image.c

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

Shut down before jumping to the multiboot image.

do it.