ttynvt: Add overall access lock

Different processes or threads accessing the device simultaneously or
rapidly after each other could cause problems.

Based on suggestion by Jo Luxer:
https://gitlab.com/lars-thrane-as/ttynvt/-/merge_requests/4

src/ttynvt.c

@@ -84,6 +84,8 @@ typedef struct {
 } ttynvt_t;
 
 
+static pthread_mutex_t access_lock = PTHREAD_MUTEX_INITIALIZER;
+
 static volatile char _is_interrupted = 0;
 
 static void sig_handler(int sig)
@@ -437,6 +439,8 @@ static void ttynvt_open(fuse_req_t req, struct fuse_file_info *info)
     int             res;
     int             n;
 
+    pthread_mutex_lock(&access_lock);
+
     nvt_log(LOG_INFO, "Connecting to server: %s:%d\n",
             ttynvt_param.host, ttynvt_param.port);
 
@@ -525,6 +529,8 @@ static void ttynvt_open(fuse_req_t req, struct fuse_file_info *info)
 
     nvt_log(LOG_ERR, "Connection to server: %s:%d failed: %m\n",
             ttynvt_param.host, ttynvt_param.port);
+
+    pthread_mutex_unlock(&access_lock);
 }
 
 static void
@@ -1135,6 +1141,8 @@ static void ttynvt_release(fuse_req_t req, struct fuse_file_info *info)
 
     nvt_log(LOG_INFO, "Disconnected from server: %s:%d\n",
             ttynvt_param.host, ttynvt_param.port);
+
+    pthread_mutex_unlock(&access_lock);
 }
 
 static const struct cuse_lowlevel_ops ttynvt_op = {