savelog: use mv -f to avoid prompting before overwriting read only files (bug #941368)

Simon Dezielrequested to merge
sdeziel-guest/debianutils:savelog-non-interactive into master

If savelog is invoked as a normal user and asked to create files that are not writeable (-m 0400), it will prompt during mv:

$ savelog -qm 0400 bar mv: replace 'bar', overriding mode 0400 (r--------)?

Closes: #941368

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