[Feat] lib/gis: Return the child exit status from G_popen_close

[wenzeslaus]

G_popen_close() waits for the child process but discards the exit status and returns void, so callers of G_popen_read()/G_popen_write() cannot tell whether the child succeeded. POSIX pclose() returns the status for this reason.

Details (analysis generated with Claude Code)

lib/gis/popen.c:

void G_popen_close(struct Popen *state)
{
    if (state->fp)
        fclose(state->fp);

    if (state->pid != -1)
        G_wait(state->pid);
}

The value of G_wait() is dropped. Returning it (with something distinguishable when there was no child) lets callers check the child, like pclose() does.

This is one of the links that made #7734 silent: r.coin runs r.stats through G_popen_read(), and when r.stats failed, the exit status was discarded here, so r.coin printed an empty table and exited with success. Analysis in #7732.