Enhancement: Add option to let backup trust mtime without checking ctime

The backup command used to require that both ctime and mtime of a file matched
with a previously backed up version to determine that the file was unchanged.
In other words, if either ctime or mtime of the file had changed, it would be
considered changed and restic would read the file's content again to back up
the relevant (changed) parts of it.

The new option --ignore-ctime makes restic look at mtime only, such that ctime
changes for a file does not cause restic to read the file's contents again.

The check for both ctime and mtime was introduced in restic 0.9.6 to make
backups more reliable in the face of programs that reset mtime (some Unix
archivers do that), but it turned out to often be expensive because it made
restic read file contents even if only the metadata (owner, permissions) of
a file had changed. The new --ignore-ctime option lets the user restore the
0.9.5 behavior when needed. The existing --ignore-inode option already turned
off this behavior, but also removed a different check.

Please note that changes in files' metadata are still recorded, regardless of
the command line options provided to the backup command.

https://github.com/restic/restic/issues/2495
https://github.com/restic/restic/issues/2558
https://github.com/restic/restic/issues/2819
https://github.com/restic/restic/pull/2823