build: more fixes and tidy-up (mostly for Windows)

- cmake: always link `ws2_32` on Windows. Also add it to `libssh2.pc`.

   Fixes #745

- agent: fix gcc compiler warning:
   `src/agent.c:296:35: warning: 'snprintf' output truncated before the last format character [-Wformat-truncation=]`

- autotools: fix `EVP_aes_128_ctr` detection with binutils `ld`

   The prerequisite for a successful detection is setting
   `LIBS=-lbcrypt` if the chosen openssl-compatible library requires
   it, e.g. libressl, or quictls/openssl built with
   `-DUSE_BCRYPTGENRANDOM`.

   With llvm `lld`, detection works out of the box. With binutils `ld`,
   it does not. The reason is `ld`s world-famous pickiness with lib
   order.

   To fix it, we pass all custom libs before and after the TLS libs.
   This ugly hack makes `ld` happy and detection succeed.

- agent: fix Windows-specific warning:
  `src/agent.c:318:10: warning: implicit conversion loses integer precision: 'LRESULT' (aka 'long long') to 'int' [-Wshorten-64-to-32]`

- src: fix llvm/clang compiler warning:
  `src/libssh2_priv.h:987:28: warning: variadic macros are a C99 feature [-Wvariadic-macros]`

- src: support `inline` with `__GNUC__` (llvm/clang and gcc), fixing:
  ```
  src/libssh2_priv.h:990:8: warning: extension used [-Wlanguage-extension-token]
  static inline void
         ^
  ```

- blowfish: support `inline` keyword with MSVC.

   Also switch to `__inline__` (from `__inline`) for `__GNUC__`:
     https://gcc.gnu.org/onlinedocs/gcc/Inline.html
     https://clang.llvm.org/docs/UsersManual.html#differences-between-various-standard-modes

- example/test: fix MSVC compiler warnings:

  - `example\direct_tcpip.c(209): warning C4244: 'function': conversion from 'unsigned int' to 'u_short', possible loss of data`
  - `tests\session_fixture.c(96): warning C4013: 'getcwd' undefined; assuming extern returning int`
  - `tests\session_fixture.c(100): warning C4013: 'chdir' undefined; assuming extern returning int`

- delete unused macros:
  - `HAVE_SOCKET`
  - `HAVE_INET_ADDR`
  - `NEED_LIB_NSL`
  - `NEED_LIB_SOCKET`
  - `HAVE_NTSTATUS_H`
  - `HAVE_NTDEF_H`

- build: delete stale zlib/openssl version numbers from path defaults.

- cmake: convert tabs to spaces, add newline at EOFs.

Closes #811

cmake/CheckNonblockingSocketSupport.cmake

@@ -86,7 +86,7 @@ int main()
       HAVE_IOCTLSOCKET)
 
       if(NOT HAVE_IOCTLSOCKET)
-	check_c_source_compiles("/* IoctlSocket test (Amiga?) */
+        check_c_source_compiles("/* IoctlSocket test (Amiga?) */
 #include <sys/ioctl.h>
 
 int main()
@@ -97,7 +97,7 @@ int main()
         HAVE_IOCTLSOCKET_CASE)
 
         if(NOT HAVE_IOCTLSOCKET_CASE)
-	  check_c_source_compiles("/* SO_NONBLOCK test (BeOS) */
+          check_c_source_compiles("/* SO_NONBLOCK test (BeOS) */
 #include <socket.h>
 
 int main()
@@ -108,12 +108,12 @@ int main()
 }"
           HAVE_SO_NONBLOCK)
 
-	  if(NOT HAVE_SO_NONBLOCK)
-	    # No non-blocking socket method found
-	    set(HAVE_DISABLED_NONBLOCKING 1)
-	  endif()
-	endif()
+          if(NOT HAVE_SO_NONBLOCK)
+            # No non-blocking socket method found
+            set(HAVE_DISABLED_NONBLOCKING 1)
+          endif()
+        endif()
       endif()
     endif()
   endif()
-endmacro()
+endmacro()