Make stderr line-buffered for logging

qrexec_logv uses several separate fprintf() calls for a single message.
Since stderr is unbuffered by default, this sometimes leads to
splitting one log message with another. Avoid the issue by turning
stderr into line-buffered mode.
Documentation for setlinebuf() (or more precisely - for underlying
setvbuf()) says it can be called only after opening the stream but
before any other operation. Since setup_logging() is called as the first
thing in main(), it should be okay.

libqrexec/log.c

@@ -88,5 +88,10 @@ void setup_logging(const char *program_name) {
             errx(125, "File descriptor %d is closed, cannot continue", i);
     }
 
+    /* qrexec_logv() uses several separate fprintf calls, print them at once */
+    errno = 0;
+    if (setvbuf(stderr, NULL, _IOLBF, 0))
+        warn("Failed to enable line buffering on stderr");
+
     qrexec_program_name = program_name;
 }