Fix problems caused by echo when the messages resembles some options or when "shopt -s xpg_echo" is set

[akinomyoga]

In general, we need to carefully use the builtin echo when the message can contain arbitrary strings.

• The echo builtin accepts options such as -n, -e, or -- (actually the set of supported options differs between shells), which means that when the message text is confusing with these options, echo behaves in a unexpected way.
• When a user sets shopt -s xpg_echo in .bashrc or set -o BSD_ECHO in .zshrc, the builtin echo processes backslashes in the message as an escape sequence, which would also cause unexpected behavior when the mark name, FZF_MARKS_FILE, or the directory paths contain backslashes.

So, when the message can contain arbitrary strings, we can use the printf '%s\n' ... idiom instead. Here is the fix.

[urbainvaes]

Many thanks, I wasn' aware that echo could cause issues. And the herestrings are much better than the previous solution! :)