When the first child of the node being validated gets deleted during

When the first child of the node being validated gets deleted during
validation, man_node_unlink() switches to MAN_NEXT_CHILD.  After
that, we have to switch back to MAN_NEXT_SIBLING after completing
validation, or subsequent parsing would add content into an already
closed node, clobbering potentially existing children, causing
information loss and a memory leak.  Bug found by kristaps@ with
valgrind in groff(7) on Mac OS X.

Note that the switch back must be conditional, for if the node being
validated itself gets deleted, we must *not* go to MAN_NEXT_SIBLING,
which would not only yield wrong results in general but also crash
in malformed manuals having an empty paragraph before the first .SH,
for example OpenBSD c++filt(1).

While here, add the missing <sys/types.h> as required before mandoc.h.