Implementing --clean-strtab

[brenoguim]

Hi,

I started to implement a simple version of --clean-strtab as suggested in #449.

Some attention points:

• I'm not sure I got all the points.
• This is really clean-dynstr so might need work to expand to other sections. But seems to be a good start for later enhancement.
• Unfortunately, for most cases --clean-strtab will actually introduce a few strings because it will undo sharing like vprintf with printf. However I still think it's valuable to have it and enhance it later on if necessary.

Maybe we should have a set of switches documented as beta due to their complexity? The name renaming and this one could be marked as experimental because it's hard to know I got all fields right.
Then after a couple of releases they could be moved to stable.

[rohoog]

I see that this pull request also does what I've done in #472. However, I don't introduce new symbols due to the symbol sharing, I just leave the sharing as-is.

[brenoguim]

I was planning to enhance it to do the sharing eventually, so it would be next to perfect.
I didn't check the size of the file in my test, only the presence of a given string.
I'll take a look at your PR @rohoog , maybe the approach of not undoing the sharing is the best.