Perl_delimcpy(): handle backslash as last char

Perl_delimcpy(): handle backslash as last char

[perl #129064] heap-buffer-overflow S_scan_heredoc
[perl #129176] Conditional jump depends on uninitialized values in
               S_scan_heredoc

Perl_delimcpy() is supposed to copy a delimited string to another buffer;
it handles \-<delimiter> escapes, but if the backslash is the last
character in the src buffer, it could overrun the end of the buffer
slightly.

Also document a bit better what this function is supposed to do.