More robust battery status handling.

Added an extra path to check for linux power supply status.
Added ignore battery option. If set to true, then when we can't figure out
the power status, we'll assume the system is plugged in.

src/daemon/rpc_command_executor.h

@@ -106,7 +106,7 @@ public:
 
   bool print_transaction_pool_stats();
 
-  bool start_mining(cryptonote::account_public_address address, uint64_t num_threads, bool testnet, bool do_background_mining);
+  bool start_mining(cryptonote::account_public_address address, uint64_t num_threads, bool testnet, bool do_background_mining = false, bool ignore_battery = false);
 
   bool stop_mining();