blowfish: build improvements

- include `blowfish.c` into `bcrypt_pbkdf.c`, instead of
  compiling it as a distinct object.

- make low-level blowfish functions static. This prevents this symbols
  to pollute the public namespace of libssh2. It also allows the
  compiler to inline these functions.

- integrate `blf.h` header into `bcrypt_pbkdf.c` as well.

- use `_DEBUG_BLOWFISH` instead of `#if 0`.

- fix `_DEBUG_BLOWFISH` compiler warnings and other nits.

- `#undef` `inline` before redefining it in `libssh2_priv.h`.
  (copied from `blowfish.c`)

- delete unused `inline` redefinitions from `blowfish.c`.

- disable unused low-level blowfish functions.

- formatting, header order.

Closes #938

src/libssh2_priv.h

@@ -120,8 +120,10 @@ int _libssh2_snprintf(char *cp, size_t cp_max_len, const char *fmt, ...);
 
 /* "inline" keyword is valid only with C++ engine! */
 #ifdef __GNUC__
+#undef inline
 #define inline __inline__
 #elif defined(_MSC_VER)
+#undef inline
 #define inline __inline
 #endif