Changes -secure mode to -sensitive, making xcmemzero default, documentation changes, bug fix

[kennbr34]

This pull implements changes discussed in the comments of https://github.com/astrand/xclip/pull/70

• '-secure' be changed to '-sensitive' and the manual more aptly describe its function
• Call XSetSelectionOwner() to set window owner to None before calling xcmemzero()
• xcmemzero() be used unconditionally to clear selection buffers upon exit
• Default '-wait' period changed from 0 to 50 milliseconds

I had some issues with my fork not being as up to date with astrand/xclip as I thought it was which accidentally reverted some other changes, but I think I have added them all back correctly. I noticed errconvsel() was returning 1 when it needed to be exiting with EXIT_FAILURE so I fixed that.

[hackerb9]

Thank you, @kennbr34. Your improvements have been merged. And thanks for catching that return versus exit error in errconvsel().