Fix linebreak (\\) command formatting.

[ddssff]

The syntax of the \ command, according to http://ftp.math.purdue.edu/mirrors/ctan.org/info/latex2e-help-texinfo/latex2e.html#g_t_005c_005c is either \[measure] or \*[measure].

[ddssff]

This is a bit drastic, but I tried outputting \%\n and did not get correct results.

[Daniel-Diaz]

With this change, the optional measure argument will always be printed. This was not happening before. Is there a reason for it?

[ddssff]

The problem is that the * character and [ or ] can appear in the plain text directly after a lnbk, and if they do they become part of the command.