Fix ESP32 encr. OTA; clean up {open,read,close}dir

Fixed a bunch of things to make OTA work on ESP32 with flash encryption:
writes to app aprtitions must be 32-byte aligned and mod 32 in size.
When merging filesystems during update, use spiffs_vfs_* functions so
that old fs is properly decrypted.

Refactored cs_dirent stuff: SPIFFS support moved to spiffs_vfs.c,
added dirent.h on ESP8266 and CC3200 which includes cs_dirent.h
Define DIR and dirent only if asked (ESP8266, CC3200, WIN32).

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