Implementing optional parameter restart to tmr:start()

[vsky279]

Fixes #3020.

• This PR is for the dev branch rather than for master.
• This PR is compliant with the other contributing guidelines as well (if not, please describe why).
• I have thoroughly tested my contribution.
• The code changes are reflected in the documentation at docs/*.

Implements the solution discussed in #3020 as wrapped up by @HHHartmann

Also removes unecessary check of lua_newuserdata result (also in ads1115 module).

[nwf]

Nits; otherwise LGTM.

[TerryE]

Lukas, I tidied up tmr.c (= quite a lot of rework) for my #3078. If you don't mind, it's going to be a lot easier to add in this couple of changes rather than having to do a three-way merge. Thanks.

[vsky279]

Of course not. Go ahead.

[vsky279]

Oups. A bit of fight with @marcelstoer in commits. I hope the result is fine.

[marcelstoer]

I hope the result is fine.

As #1168 is now closed we maybe shouldn't argue about it but replacing spaces with tabs is a move in the wrong direction IMO.

I'm happy to merge this though.

[vsky279]

spaces -> tabs - I only made it consistent with the rest of the file where tabs are used.
In the following PR we can replace tabs to spaces for good.