vfs and newlib: small fixes

- spaces->tabs in tasks.c
- update vfs_uart.c to use per-UART locks
- add license to vfs_uart.c
- allocate separate streams for stdout, stdin, stderr, so that they can be independently reassigned
- fix build system test failure
- use posix off_t instead of newlib internal _off_t

components/vfs/vfs.c

@@ -167,7 +167,7 @@ ssize_t esp_vfs_write(struct _reent *r, int fd, const void * data, size_t size)
     return ret;
 }
 
-_off_t esp_vfs_lseek(struct _reent *r, int fd, _off_t size, int mode)
+off_t esp_vfs_lseek(struct _reent *r, int fd, off_t size, int mode)
 {
     const vfs_entry_t* vfs = get_vfs_for_fd(fd);
     if (vfs == NULL) {