Some upcoming code wants to serialize JSON into a "struct ds" dynamic
string buffer, so expose an interface to do this.
This commit doesn't change much, but it renames some functions internal
to json.c to make the naming more consistent.
Also, make jsonrpc_log_msg() use this new function, since it is a more
straightforward way to do what it wants.
The JSON syntax for strings is very reasonable. An upcoming commit will
have a need for a string parser, so make the JSON string parser available
for that.
Also, this change improves the error message for strings that end in the
middle of a \u sequence, so update the tests to match.
The chroots in which we often build Open vSwitch don't have /proc and
thus cannot support /dev/stdin, because on Linux that is a symlink to
/proc/self/fd/0. So avoid using /dev/stdin in the testsuite.
