Additional useful capabilities

[qouoq]

Following #233, here's an update for main.cpp (error message and comment), and the man page.

A first important point is that the error message was not shown on the console. The unexpected behavior was introduced with #231 as far as I can tell. I have no experience of ncurses, but my feeling is that initializing it (calling init_ui) before outputting to stderr/stdout messes up the console output. My tentative solution is to defer initialization to just before main loop (lines 301-304 in this PR), which in my tests works well for error and normal cases. (Before #231, behavior was also messy, in the sense that newlines were not displayed properly in the console.)

The second point is that the long setcap command line in the man page looks quite ugly. I quickly tried to look at groff macros, but that looks like a lot of complicated stuff, so trying to find an elegant way to display that is not something I can help much with I'm afraid..

[raboof]

LGTM, but I won't merge since you still have it marked as 'Draft'

[qouoq]

Alrighty, I couldn't find a more elegant way to handle this long line in the man page.