Use appendStringInfoString() where appropriate in elog.c.

The nominally equivalent call appendStringInfo(buf, "%s", str) can be
significantly slower when str is large.  In particular, the former usage in
EVALUATE_MESSAGE led to O(N^2) behavior when collecting a large number of
context lines, as I found out while testing recursive functions.  The other
changes are just neatnik-ism and seem unlikely to save anything meaningful,
but a cycle shaved is a cycle earned.