Various small chan_skinny fixes and cleanup

Added test to skinny_register to only allow device to register against
a device that is not already registered.

Addback l->device test for skinny_show_lines. Fixes segfault if a line
is configured but not configured to a device. Reverses part of r368680.

Removed redundant l->device tests in subsubstate and dumpsub. l->device
will always be valid if these routines are called. Reverses 368948 -
discussed with mjordan on irc.

Some indentation cleanup.


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@369034 65c4cc65-6c06-0410-ace0-fbb531ad65f3