collectd: Don't *abort* on the first read-error with the `-T' option.

collectd: Don't *abort* on the first read-error with the `-T' option.

The `-T' option used to basically quit the daemon right away when the
first read function of a plugin failed. This patch changes the behavior,
so that:

 - All read-functions are tried. If one or more fail, the daemon will
   exit with a non-zero exit status, but all read-functions will be
   tried.
 - Don't quit if one read-function failed without calling all the
   shutdown-functions first. This will clean up the UNIX socket of the
   unixsock plugin and stuff like that.