Correctly handle L'\\'

Hannes filed a bug that pointed out that:

    print L'\\'

... did not work correctly.  The bug is in convert_escape, which
simply transcribes the backslash character, rather than convert it
between encodings.

This patch fixes the error.  I also turned a macro into a lambda to
clean up this code a little.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=33124
Reviewed-By: Tom de Vries <tdevries@suse.de>
Tested-By: Hannes Domani <ssbssa@yahoo.de>