fix dual fire handling for banks that cannot use it

[Goober5000]

The Secondary_dual_fire flag is per-ship, but not every secondary bank can dual fire. Previously the HUD drew the dual fire link icon even when the armed bank had only one firepoint, and firing such a bank cleared the flag entirely, silently discarding the player's dual fire preference for the other banks.

Extend the ignored-rather-than-cleared approach already established for no-doublefire weapons to the single-firepoint case:

• add ship_secondary_bank_can_dual_fire() as the single source of truth for bank validity, No_doublefire, ai_profiles restrictions, and firepoint count
• ship_fire_secondary() now fires single and leaves the flag alone when the bank cannot dual fire, instead of removing the flag
• the weapons gauges and the hardpoints gauge consult the helper, so the link icon and second-firepoint highlight are no longer drawn for incapable banks
• the dual fire toggle on a single-firepoint bank now explains why it cannot dual fire and leaves the preference untouched
• restoring the saved dual fire preference no longer depends on bank 0's firepoint count, since incapable banks now simply ignore the flag
• weapon_create()'s substitution pattern logic also consults the helper so the pattern index no longer advances as if a pair were fired