[freeciv-ai] ai bodyguard code cleanup part#1

["Per I. Mathisen" <per@xxxxxxxxxxx>]

This patch does stylistic cleanup and introduces the new (but here unused)
function ai_unit_new_role(), which should add an incremental number of
much needed sanity checks to unit task assignment in the AI code.

Cleanups:
 - new enum BODYGUARD_* instead of magic numbers -1, 0
 - documented several functions
 - cleaned up style in look_for_charge and ai_military_bodyguard

There are no autogame changes. Please proofread.

More bodyguard patches will be coming, since I want to see if I can give
our active diplomats bodyguards.

Yours
Per

bodyguardclean1.diff
Description: Text document