(bp): Rename member val to bp_item, member avail to bp_avail. Users

(bp): Rename member val to bp_item, member avail to bp_avail.  Users
changed.  Switch from int to short, to match struct sctstr members
sct_item and sct_avail.

Symbolic indexes for struct bp member bp_item[]:
(BP_NONE, BP_CIVIL, BP_MILIT, BP_SHELL, BP_GUN, BP_LCM, BP_HCM)
(BP_MAX): New.
(bug_key): Use them.  Values are now array indexes, not indexes + 1.
(bp_get_item, bp_put_item, bp_set_from_sect): Update for changed
bud_key[].