upscli_readline - read a single response from a UPS
int upscli_readline(UPSCONN_t *ups, char *buf, size_t buflen);
The upscli_readline() function takes the pointer ups to a
UPSCONN_t state structure, receives a single line from the server, and copies
up to buflen bytes of the response into the buffer buf.
Some parsing of the string occurs during reception. In particular,
ERR messages from upsd(8) are detected and will cause this function
to return -1.
The upscli_readline() function returns 0 on success, or -1 if an error