Always use FILE* for standard output.

The threads running commands for clients through the Posix daemon used
to write to a char buffer through snprintf (etc.) which was then written
directly to the file descriptor, whereas in the other case printf
(etc.) was used to write to stdout (FILE*). Both versions used some
macro's and repeated code to have the same output.

This change unifies these two cases by using a FILE* in both cases. The
(line) buffering is done by the standard C library's implementation
(just like with stdout), and px4_log.c now uses the same code in all
cases (using fprintf, etc.) for printing (colored) output.

src/systemcmds/tests/test_mixer.cpp

@@ -40,6 +40,7 @@
 #include <limits>
 #include <dirent.h>
 #include <string.h>
+#include <unistd.h>
 
 #include <px4_config.h>
 #include <mixer/mixer.h>