2011-09-13  Abhijit Halder  <abhijit.k.halder@gmail.com>

	Modify logging.

	* main.c (gdb_logfile): Define a new global variable.
	* defs.h (gdb_logfile): Of type GDB_FILE, will pass this around gdb
	to access the log file if presents.
	* top.c (execute_command): Log the command to be executed on file if
	logging is enabled.
	* cli/cli-logging.c (handle_redirections): Assign gdb_logfile the file
	streams associated with log-file.
	(pop_output_files): Reset gdb_logfile to NULL.