Remove atcommand_info's first (redundant) entry

This was an experimental in pair programming. :)
Fix stupid crash on bad @command
[magic] Add mob_master_id() and pc_id().
[magic-expr.cpp] remove pc_id(), which is duplicated with hash_entity().
[magic] Add mob_master_id() and pc_id().
[magic-expr.cpp] remove pc_id(), which is duplicated with hash_entity().
[magic-expr] rename script_int() to get_script_variable().