output_filter() does not need to dot-terminate its input name because
libidn2 properly handles both dot-terminated and non-dot-terminated
names.  libidn2 also does not implicitly dot-terminate names passed to
it, so parts of output_filter() handling dot termination can simply be
removed.

Fix a logical condition to make sure 'src' can fit the terminating NULL
byte.  Replace the MAXDLEN macro with the MXNAME macro used in the rest
of dig source code.  Tweak comments and variable names.

Rename output_filter() to idn_output_filter() so that it can be easily
associated with IDN and other idn_*() functions.