Revert "Revert "Fixed calculation of capacity on OpenBSD (#441)""

This reverts commit 681b22d5c7.

The statvfs man page:
http://man7.org/linux/man-pages/man3/statvfs.3.html

says clearly:

fsblkcnt_t     f_blocks;   /* Size of fs in f_frsize units */

I missed this earlier.

src/nnn.c

@@ -3281,7 +3281,7 @@ static size_t get_fs_info(const char *path, bool type)
 		return 0;
 
 	if (type == CAPACITY)
-		return svb.f_blocks << ffs((int)(svb.f_bsize >> 1));
+		return svb.f_blocks << ffs((int)(svb.f_frsize >> 1));
 
 	return svb.f_bavail << ffs((int)(svb.f_frsize >> 1));
 }