- Find the the absolute path of the executable and make argv[0] point to it

- Deduce mysql_home (aka basedir) from the executable path

- Let mysqld::runtime::mysqld_daemonize() return (rather than call
  exit directly) on the parent side, and call
  flush_error_log_messages() to get all messages from the daemonize
  process properly formatted. Move the call to mysqld_daemonize() to a
  point where paths have been resolved agains CWD so that a daemon
  process treats relative paths in the same way as a regular server.

- Print the name of the chosen error log file before opening and redirecting
  stderr so that users can see where the daemon will try to log messages.

- Fix a bug in open_error_log() which assumed the log file could be used if stat
  showed it to be writable for user (that will be true even if it is not owned
  by the current uid). When calling my_freopen() on such a file the stderr
  FILE pointer becomes invalid, and this causing messages to be lost.

- New mtr test main.basedir.