libc: Implement ttyname and ttyname_r

Note: this patch can get file path from root pseudo file handle,
but a general infrastructure is setup for other file system too.

Signed-off-by: Xiang Xiao <xiaoxiang@xiaomi.com>
Change-Id: I86cd79ebb741f2f43fdd398bb7498c40687d949b

include/unistd.h

@@ -322,6 +322,9 @@ int     ftruncate(int fd, off_t length);
 /* Check if a file descriptor corresponds to a terminal I/O file */
 
 int     isatty(int fd);
+
+FAR char *ttyname(int fd);
+int       ttyname_r(int fd, FAR char *buf, size_t buflen);
 #endif
 
 /* Memory management */