Bug fix: X error handler & Use NameWindowPixmap & Misc

- Add useNameWindowPixmap support as a workaround to a bug in nVidia's X
  Render implementation when doing transform on window pictures. Not
  enabled by default.

- Add session-wide structure session_t to store options and X extension
  info. It could be a part of MainWin however MainWin is created too
  late.

- Pre-parse all configuration options for efficiency. Add configuration
  parsing helper functions config_get_{bool,int,double}(). Note because
  there are just too many options, chances are I made some errors about
  max/min/default values of those options.

- Add X error handler to prevent application from exiting on X Error.
  (upstream issue 9)

- Move extension queries to init_xext() in skippy.c.

- Explicitly initialize SkippyWindow and ClientWin to all 0.

- Use getopt_long() to parse commandline arguments. Add -S for X
  synchronization (for debugging).

- Revise configuration file path detection to adhere to XDG standard.
  Add `make install` rules to install skippy-xd.rc-default to
  /etc/xdg/skippy-xd.rc . (upstream issue 10)

- Rename macro XINERAMA to CFG_XINERAMA, DEBUG to DEBUG_XINERAMA.

- Add convenience functions allocchk(), free_picture(), free_pixmap(),
  free_region(), free_damage(), print_timestamp(), and macro printfe(),
  printfef().

- Properly declare WM atom variables to silence warnings.

- Misc changes.