quick-wins: ControlMaster, -z alias, --progress, fix write warning

- SSH ControlMaster auto with ControlPath for connection reuse
- -z as alias for -c (compression)
- --progress flag showing transfer rate in send_files
- Fixed unused-result warning on write() in transport_ssh.c

src/shared/config.c

@@ -22,6 +22,7 @@ Config *config_create(char *version, char *send_directory,
   config->use_chunk_serialization = use_chunk_serialization;
   config->use_compression = use_compression;
   config->use_metadata = use_metadata;
+  config->show_progress = false;
   config->compression_level = compression_level;
   config->use_sendfile = use_sendfile;
   config->chunk_size = chunk_size > 0 ? chunk_size : DEFAULT_CHUNK_SIZE;

src/shared/config.h

@@ -18,6 +18,7 @@ typedef struct Config {
   bool use_compression;
   bool use_sendfile;
   bool use_metadata;
+  bool show_progress;
   int compression_level;
   unsigned long long chunk_size;
   TransportType transport;

src/shared/transport_ssh.c

@@ -91,10 +91,12 @@ Client *client_connect_ssh(char *destination) {
       snprintf(ssh_user, sizeof(ssh_user), "%s", r.host);
 
     execlp("ssh", "ssh", "-o", "Compression=no", "-o",
-           "ControlMaster=no", ssh_user, "fastsync-server", "--stdio",
-           (char *)NULL);
+           "ControlMaster=auto", "-o",
+           "ControlPath=~/.cache/fastsync-%r@%h:%p", ssh_user,
+           "fastsync-server", "--stdio", (char *)NULL);
     perror("exec of ssh failed");
-    (void)write(exec_pipe[1], "x", 1);
+    ssize_t wret = write(exec_pipe[1], "x", 1);
+    (void)wret;
     _exit(1);
   }