tidy-up: misc

- lib1531: fix to use `curl_off_t`.
- src/tool_doswin: merge `_WIN32` guards.
- src/tool_doswin: limit `S_ISCHR()` to MS-DOS.
- src: dedupe `CURL_O_BINARY` macro into `tool_setup.h`.
- CMake/CurlTests.c: cleanup/formatting `HAVE_FILE_OFFSET_BITS` test.
- examples/httpput-postfields: delete unused include.
- spelling: MultiSSL.
- whitespace.

Ref: #15652
Closes #15799

src/tool_operate.c

@@ -111,14 +111,6 @@ extern const unsigned char curl_ca_embed[];
 #endif
 #endif
 
-/* since O_BINARY is used in bitmasks, setting it to zero makes it usable in
-   source code but yet it does not ruin anything */
-#ifdef O_BINARY
-#define CURL_O_BINARY O_BINARY
-#else
-#define CURL_O_BINARY 0
-#endif
-
 #ifndef SOL_IP
 #  define SOL_IP IPPROTO_IP
 #endif