Block closing macros do not allocate a new node but finish an existing

Block closing macros do not allocate a new node but finish an existing
one, so they miss the clearing of MDOC_NEWLINE in mdoc.c, node_alloc().
Consequently, MDOC_NEWLINE must be cleared before processing the next
macro on the same line.
This fixes horizontal spacing for input lines beginning like .Oc Ns ...

Issue found by Franco Fichtner <franco at lastsummer dot de>
while working on DragonFly mandoc integration.