helpers: add run_pty() function

This runs an external command inside a pty. Running inside a pty,
external commands using stdio(3) will disable stdout buffering when
running from a terminal, which is often desirable.

src/system/helpers.h

@@ -41,6 +41,7 @@ int my_system(const char * cmd);
 int my_system(int argc, const char *arg, ...); /* argc is number of arguments including command */
 
 FILE* my_popen( pid_t& pid, const char *cmdstring, const char *type);
+int run_pty(pid_t &pid, const char *cmdstring);
 
 int safe_mkdir(const char * path);
 inline int safe_mkdir(std::string path) { return safe_mkdir(path.c_str()); }