xmldoc: Improve xmldoc wrapping of 'core show ...' output.

Previously, the wrapping did both lookahead and lookback, which,
together with color escape sequences, caused some lines to be wrapped
way earlier than other lines.  This led to inconsistent output.

This simplifies the wrapping code and makes it more sane: if maxcolumns
is hit, we simply jump back to the last space and wrap there.

ASTERISK-25527 #close

Change-Id: I56d01c6f9a812642b1b05535c98d4db48d17c957