Fix VLAN name persistence across rename and delete operations

Previously, renaming a VLAN or deleting and recreating it with a
different name did not update the displayed name. The vlan_names[]
array is an append-only buffer where vlan_name() returns the first
matching entry, so stale entries kept winning.

This commit adds vlan_name_remove(), which locates an entry by
VLAN ID and removes it via array compaction. The function is called
in two places:

  - parse_vlan() in cmd_parser.c, before appending a new name entry,
    to remove any pre-existing entry for the same VLAN ID
  - vlan_delete() in rtl837x_port.c, to clean up the name when a
    VLAN is removed

The implementation reuses the existing vlan_name() lookup, scans for
the trailing space of the matched entry, then shifts remaining bytes
left. Locals are declared as static __xdata to avoid the SDCC
overlay segment limit on banked functions.

Tested on KeepLiNK KP-9000-6XH-X:
  - vlan 99 AAA p1u; vlan 99 BBB -> name updated to BBB
  - vlan 99 d; vlan 99 CCC p1u   -> name correctly CCC, not stale AAA

Note: This fix addresses the runtime XMEM state. Persistence of
renamed VLAN names across reboot requires the user to download and
re-upload /config, as is the existing pattern for all configuration
changes in this firmware.

rtl837x_port.h

@@ -50,6 +50,7 @@ void port_l2_learned(void) __banked;
 void port_stats_print(void) __banked;
 int8_t vlan_get(register uint16_t vlan) __banked;
 __xdata uint16_t vlan_name(register uint16_t vlan) __banked;
+void vlan_name_remove(uint16_t vlan) __banked;
 void vlan_setup(void) __banked;
 void port_pvid_set(uint8_t port, __xdata uint16_t pvid) __banked;
 uint16_t port_pvid_get(uint8_t port) __banked;