Truncate the file behind the given channel at the given location (or the current by default). Operation is (probably) not supported by non-file channels.
Like the ftruncate() call in C's stdio.
Introduced in Tcl 8.5.
AMG: For one possible use of this command, see How do I remove one line from a file?.
AMG: Any plans to add support for [chan truncate] to reflected channels?