pgsql: Fix latent bug in ExecSeqRestrPos: it leaves the plan node's

Log Message:
-----------
Fix latent bug in ExecSeqRestrPos: it leaves the plan node's result slot
in an inconsistent state. (This is only latent because in reality
ExecSeqRestrPos is dead code at the moment ... but someday maybe it won't
be.) Add some comments about what the API for plan node mark/restore
actually is, because it's not immediately obvious.

Modified Files:
--------------
pgsql/src/backend/access/index:
indexam.c (r1.80 -> r1.81)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/index/indexam.c.diff?r1=1.80&r2=1.81)
pgsql/src/backend/executor:
execAmi.c (r1.83 -> r1.84)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/executor/execAmi.c.diff?r1=1.83&r2=1.84)
nodeMergejoin.c (r1.73 -> r1.74)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/executor/nodeMergejoin.c.diff?r1=1.73&r2=1.74)
nodeSeqscan.c (r1.52 -> r1.53)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/executor/nodeSeqscan.c.diff?r1=1.52&r2=1.53)