Carefully let mandoc_getcontrol() handle libroff's detection of a macro

Carefully let mandoc_getcontrol() handle libroff's detection of a macro
line.  Note that we now properly ignore \.", which earlier would have
been pushed through macro detection.  This required attention to all
entries of roff_parse().

This removes a TODO by schwarze@ to the effect that \. can be a control
character, which is now handled in mandoc_getcontrol().