Fix very rare crash on :chmod

Operating on local buf buffer contained a bug.  Part of string, which
separates files was not included to line length.

Thanks to Marton Balazs, a.k.a. balmar.
Fix chmod on files with long names

They were truncated and appended by ellipsis.
Change the way title of the perms dialog is made

Use file name if only one name is processed.
Use "n files" for multiple files.

Before file under the cursor was displayed, even if it wasn't affected
by chmod operation.
Plan reducing duplication of undo msg composing
Merge branch 'fix-chmod-crash'