More cleanup in lex.pl

- Removed an over-prominent log message in tagdb_util.c
- Deleted legacy code in tagfs.lc
Modified logging system

- Log system now makes use of macros for file-based log level. This
  allows the same level of granularity and flexiblity as before, but
  without the troublesome use of behind-the-scenes assignment of global
  variables
  * Also added a command line option to specify the log file name
  * Removed references to _log_level outside of log.c (it was renamed
    anyway)
- The root directory is now detected
- Commented out references to search_fs in makefile and in tagfs.lc
  (uses some older stuff)
- Turned references to a lex.pl-generated tagfs_operations struct into a
  macro %%%operatons_struct_name%%% for better separation
- Rewrote (proc_options => process_options) to use the glib option parser.
  Uses global config variables (prefixed with c_).
- Removed storage of the mount directory in our tagfs_data; it makes
  processing options less stupid and we didn't need to store it anyway.