Merge branch 'blackhole-stdout-when-piping' into master

Signed-off-by: Matthias Beyer <mail@beyermatthias.de>

Fix: Return actual length of buffer

The function has to return the number of bytes in the buffer, so return
the length of the buffer here.
Still, the contents are ignored and not written to any destination.

Signed-off-by: Matthias Beyer <mail@beyermatthias.de>

Update copyright years to 2015-2020

Signed-off-by: Matthias Beyer <mail@beyermatthias.de>

Update copyright years

Signed-off-by: Matthias Beyer <mail@beyermatthias.de>

Merge branch 'minor'

This merges some important fixes:

* libimagstore: Remove calls to filesystem-accessing functions but use
  abstractions instead.
* Make Debug for FileLockEntry more verbose
* Fix In-Memory test backend bug where the backend did not remove the
  old entry on "move"

as well as some nice formatting stuff and refactorings to simplify code
and similar improvements.

Signed-off-by: Matthias Beyer <mail@beyermatthias.de>

Remove unnecessary scope

Signed-off-by: Matthias Beyer <mail@beyermatthias.de>

Merge pull request #1325 from matthiasbeyer/libimagrt/locked-io

libimagrt: locked io

Provide LockedOutputProxy which holds locked variants of Stdout/Stderr

Let the OutputProxy hold Stdout/Stderr objects, so we do not have to aquire them each write!()

Merge pull request #1315 from matthiasbeyer/libimagrt/provide-io

libimagrt: provide io

Implement proxy object where runtime configures output

This is another approach for providing access to stdin/out/err via
libimagrt::runtime::Runtime.

The Runtime object does configure which output gets returned (stdout if
stdout is a tty, else stderr).

With this we can change libimagrt to read/write the store from/to
stdin/stdout without the user noticing that she does not write to stdout
but stderr.
Reading from stdin is not possible then, though.