Simplify checksum generation on the TX datapath.

Move routing into a separate function

produces slightly smaller code.

Truncate TX length to TCP window at time of transmission rather than at
time of adding to TX packet; this is conceptually cleaner and also allows
the application to call tcp_send() multiple times to build up a single
packet.

"flags that are currently being sent".  This allows at least one special
case (checking that we haven't already sent a FIN in tcp_rx_fin()) to be
collapsed.

reopen connection.  (The TCP API will soon change to render this step
unnecessary.)

TargetAddress from the login response, because we still need the old
address while we close the connection!

phase.  In iscsi_issue(), detect this state and immediately refuse the
operation.  This avoids trying multiple logins when scsi.c tries
several times to read the drive capacity.

"Permission denied" as an error text to strerror().

anything over 0x80 is an extended character.

curses.h so that you don't have to suffer the curses namespace pollution
just to get the symbolic key names.

causing the serial console to ignore input, because it happened to end up
linked with serial_ischar() at address 0, which core/console.c decided was
invalid).

Kill errortab array; it can screw up alignment.

"boot" command to attempt booting from within the command shell, fall back
to shell if boot fails for any reason.

screen sequence doesn't work (like bios_console.c on some platforms).