Refuse to push a non-regular file server

If SCRCPY_SERVER_PATH points to a directory, then a directory will be
pushed to /data/local/tmp/scrcpy-server.jar.

When executing it, app_process will just abort and leave the directory
on the device, causing scrcpy to always fail.

To avoid the problem, check that the server is a regular file before
pushing it.

Closes #956 <https://github.com/Genymobile/scrcpy/issues/956>

app/src/command.c

@@ -4,6 +4,9 @@
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
+#include <sys/types.h>
+#include <sys/stat.h>
+#include <unistd.h>
 
 #include "config.h"
 #include "common.h"
@@ -202,3 +205,14 @@ process_check_success(process_t proc, const char *name) {
     }
     return true;
 }
+
+bool
+is_regular_file(const char *path) {
+    struct stat path_stat;
+    int r = stat(path, &path_stat);
+    if (r) {
+        perror("stat");
+        return false;
+    }
+    return S_ISREG(path_stat.st_mode);
+}