Raise the required level to dip for Excalibur

[copperwater]

Raise XL cutoffs for Excalibur higher; possibly much higher for non-Knight lawfuls.

• lv 10 for knights, 15 for others?
• cf. fourk excalibur bias towards knights (f9b0c65) which makes odds scale
• evilhack allow only knights to dip for excalibur (possibly all lawfuls above XL15) (1a73b41)
• #2423: Only knights of XL 7 or higher can dip for Excalibur, and no other characters can, though it remains the lawful crowning gift.
• #733: Raise the XL threshold of dipping Excalibur for non-Knight roles to around 9-12. Knights can still dip at level 5.
  • This isn't necessarily the Excal change that's desired. As it's a crowning gift, perhaps the level minimum should be raised for all roles, with Knights getting some different benefit.
• If implemented this needs to have some message for when a lawful XL5+ character ineligible for Excalibur dips into a fountain and otherwise would have got it, indicating they are not experienced enough.
• Reasoning for this feature (for commit msg) is that Excal is a prime example of a feature balanced for not many players actually knowing about it, similar to Elbereth. Very powerful but not easy to get by accident. In the age of nethackwiki, this doesn't work anymore.
• There is weirdness now for Knights, in that Arthur has Excalibur when you enter the Quest, which then blocks you from getting it in any way short of stealing it from him. Likely fix (until Knight quest redesign) is to remove Arthur getting this, so that it doesn't make life unnecessarily hard for Knights.