cmdline: bunch of fixes related to config file selection

- fix potential memory leak of system_path

  'Potential' because currently g_get_system_config_dirs()
  returns single entry on Windows, but that might change.

- remove incorrect g_free() call

  It's not required at all because
  g_get_system_config_dirs() returns GLib owned memory.

- remove extra semicolon
mpd_auto.m4: Pass libraries to AC_CHECK_LIB in MPD_AUTO_PKG_LIB

Rationale: vanilla libid3tag does not have any pkg-config stuff
and fails to detect because symbols from libz are not found.
cue_parser: fix memory leak
.gitignore: add more debug programs
cue_parser: add code comments
cue_parser: fix CUE files with only one track

Track whether _finish() has been called, and deliver all partial
results then.  Fixes Mantis ticket 0003621.