cut: correct delimiter usage in regex

[mknos]

• A backslash is a valid field delimiter for cut, but it caused an error here because it was unintentionally interpreted into the regex
• Some existing scripts use \Q & \E pair in the regex to avoid this kind of problem; do it here too
• This was found when testing against other versions

Comparison with Linux system cut(1), before patch.

% echo "yo\\1" | cut -d\\ -f 1
yo
% echo "yo\\1" | perl -Mdiagnostics cut -d\\ -f 1
Trailing \ in regex m/\/ at cut line 155, <> line 1 (#1)
    (F) The regular expression ends with an unbackslashed backslash.
    Backslash it.   See perlre.
    
Uncaught exception from user code:
	Trailing \ in regex m/\/ at cut line 155, <> line 1.
	main::handle_f(1, "\\", undef) called at cut line 38