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