httpd: reuse the slot of a valid cookie, end sessions on passwd

Review feedback on #560 (DrDoof):

- A login that came with a cookie that is still valid took a second
  slot. The first one looked fresh, so the next login could push out
  someone who is using the switch. The slot of the cookie now gets the
  new id.
- Changing the password leaves no old login valid: session_init() from
  parse_passwd(). With one session the next login pushed a leftover one
  out, with several it would stay valid until the next restart.
- The 32-bit comparison of the last use times moves into a leaf function,
  so the compiler does not give it a 4-byte scratch in internal RAM.

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

cmd_parser.c

@@ -24,6 +24,7 @@
 #include "sfp.h"
 #include "dhcp.h"
 #include "syslog.h"
+#include "sessions.h"
 #include "dns.h"
 #include "ntp.h"
 #include "uip/uip.h"
@@ -1479,6 +1480,9 @@ void parse_passwd(void)
 			passwd[j++] = c;
 		} while (c != NUL && j < 20);
 		passwd[j] = NUL;
+		/* Logins made with the old password end here. With one session the next
+		 * login pushed a leftover one out; with several it would stay valid. */
+		session_init();
 		return;
 	}
 	cmd_error("Missing password\n");