httpd: keep the login sessions in bank 3

The web server's bank had no room left for the four sessions once the
original firmware's more robust header handling came in. Picking a slot,
checking a cookie and starting a session move to sessions.c in bank 3;
httpd.c only calls session_init(), session_check() and session_new().
The random id is written straight into the reply, which drops
gen_random_hex_chars() from bank 1.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Makefile

@@ -105,6 +105,7 @@ SRCS = \
 	sfp.c \
 	dns.c \
 	dns_cli.c \
+	sessions.c \
 	ntp.c \
 	ntp_cli.c \
 	syslog.c \