Several improvements in editor saving behavior

By far the most frustrating thing in Midnight Commander for me was when I accidentally start editing a file in `/etc` while running mc as non-root, and then end up in a situation where I either have to discard my changes, re-run mc as root, and redo them, or save the file to my home directory and then manually `sudo cp` it over the destination. This patch solves the problem along with some others:

- OS errors are propagated to the user - editor errors now display `errno` (text and number), like the file manager
- Some spurious "The file has been modified in the meantime" messages were fixed
- When failing to save the current file, show an error message instead of displaying the "Save as..." dialog
- When failing to save a file (and displaying the reason why), allow the user to select "Save as..." or "Cancel"
- Additionally, if the user is not root, and the file failed to save due to an access or permission error, give the user the option "sudo", which will save the file to `mc_tmpdir` and run `sudo cp <tmpname> <target>` in the subshell. This allows the user to authenticate themselves to `sudo` and complete the file save.

If there are any issues with the patch, I'll be happy to address them and rebase the pull request.