Add "x" prefix to calls to plain malloc(), calloc(), strdup(), realloc().

In review commit 503f53bfdde "Read dictionary encoding from data files"
I noticed uses of plain strdup() (not xstrdup()).  Some greps showed that
there were several other uses of strdup(), as well as calloc(), malloc(),
and realloc(), in the source tree.  This commit adds "x" prefixes to each
of them, to ensure proper error handling.