feat: libc - add strndup, scandir, alphasort, usleep, ftruncate, M_PI constants

Co-Authored-By: Claude Opus 5 <[email protected]>
Claude-Session: https://claude.ai/code/session_018Ae69wJS7sueQMUwNxV3nX

programs/include/libc/unistd.h

@@ -61,6 +61,8 @@ long   sysconf(int name);
 extern char **environ;
 
 unsigned int sleep(unsigned int seconds);
+int    usleep(unsigned long usec);
+int    ftruncate(int fd, long length);
 
 #ifdef __cplusplus
 }