fix tail input seeking when used with pipe

[phemmer]

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Fixes the tail input plugin so that it works with named pipes.
Also fixes errors being discarded when tailer dies.

There is however still one problem with reading from a pipe. If there is nobody has the pipe open for writing, when telegraf opens it for read, it'll block.
The simple solution to this is to create the goroutine before opening the file. However this means that the Start() func will return, and then the goroutine potentially error and die immediately due to some other reason (e.g. permission errors). However we can use the Accumulator.AddError() method to report the error. Not sure if there would be any end-user noticeable effect from such a change.

Note that there appears to be a different error in that if the tailer dies, it doesn't start back up, thus requiring a restart of telegraf. But that's out of scope of this issue.

Fixes #2089