mender-artifact: set default file name (-o) based on artifact name (-n)

There is some confusion between the Artifact name (meta data) and output file name (see below).

Acceptance criteria, for mender-artifact:

• If -o is not provided, then what is input to -n (processed to ensure valid file name, see below) becomes the default to -o
  • The file name is pre-processed to be safe and valid, so no special characters or glob patterns are part of the file name, even if those are allowed in -n. ".mender" is always appended to the file name
• If -o and -n is provided, then it behaves as today (they are independent)
• Bump mender-artifact major version as someone may be depending on the default file name (I think it's just "artifact.mender")

When running 

./mender-binary-delta-generator --help

It gives:

...
ARGUMENTS:
...
-n, --artifact-name NAME
...
-o, --output-path PATH
...
XDELTA3 OPTIONS:
...
-n Disable checksum
...

If you try to use -n , it will not work for naming, -artifact-name doesn't work either.

Using -o for naming the new file works fine. Even it is used in the example from this docs:

https://docs.mender.io/artifact-creation/create-a-delta-update-artifact 

So at this point one of these scenarios could be happening:

• Option 1) The naming flag -n has a bug so we need to fix it

• Option 2) The --help option gives wrong information, so we need to fix it.

Side comment, it is difficult to understand why the -n is duplicated and how to use it properly.