patch: garbage error strings in libpq

Several libpqxx users have been reporting odd problems with certain error
messages generated by libpq. One of them was the inclusion of garbage
data.

As it turns out, src/interfaces/libpq/fe-misc.c contains several instances
of this construct:

printfPQExpBuffer(&conn->ErrorMessage,
libpq_gettext("error: %s"),
SOCK_STRERROR(SOCK_ERRNO, buffer, sizeof(buffer)));

This may occur in other source files as well. On Unix-like systems,
SOCK_ERRNO defines to plain errno--which is likely to be overwritten by
the libpq_gettext(). I'm attaching a patch that fixes these instances by
introducing a named pointer to the SOCK_STRERROR message, initialized
before either of the other function calls.

Another approach would have been to make libpq_gettext() preserve errno.
It's tempting, but I'm not sure it would be valid from a language-lawyer
point of view. There is no sequence point between the evaluations of
libpq_gettext() and SOCK_STRERROR(). From what I vaguely remember hearing
somewhere in the distant past, that means that theoretically they may be
evaluated not just in any order but even in parallel. I guess it may
actually happen if both inlining and scheduling are sufficiently
aggressive. Even if libpq_gettext() is made to restore errno, it will
still have to pollute errno at some points during its execution.

Jeroen