Attachments: tweak selection, fix #NO content-description..

Attachments: tweak selection, fix #NO content-description..

Sofar when a `#NO' message attachment was replaced with a real
file attachment (by editing the name, not by deselection + new
selection) some attributes of the old attachment were not reset.

The entire interactive attachment filename + charset selection
"dialog" is now interruptible via ^C, though the interrupt is yet
propagated up blindly instead of differentiating in between
"during filename" and "during charset selection" input.

Interrupting will actually act as if the current attachment has
been removed, while keeping later ones intact.
However, all of this is an incredibly ugly hack, with patched-upon
signal handlers and siglongjmp()s and all of those terrible things
which make this codebase a horrific one.
Most of it will need a rewrite when the signal-manager solution is
implemented, but until then we have at least some options now.
Items have been added to TODO accordingly.