tidy-up: delete parallel/unused feature flags

Detecting headers and lib separately makes sense when headers come in
variations or with extra ones, but this wasn't the case here. These were
duplicate/parallel macros that we had to keep in sync with each other
for a working build. This patch leaves a single macro for each of these
dependencies:

- Rely on `HAVE_LIBZ`, delete parallel `HAVE_ZLIB_H`.

  Also delete CMake logic making sure these two were in sync, along with
  a toggle to turn off that logic, called `CURL_SPECIAL_LIBZ`.

  Also delete stray `HAVE_ZLIB` defines.

  There is also a `USE_ZLIB` variant in `lib/config-dos.h`. This patch
  retains it for compatibility and deprecates it.

- Rely on `USE_LIBSSH2`, delete parallel `HAVE_LIBSSH2_H`.

  Also delete `LIBSSH2_WIN32`, `LIBSSH2_LIBRARY` from
  `winbuild/MakefileBuild.vc`, these have a role when building libssh2
  itself. And `CURL_USE_LIBSSH`, which had no use at all.

  Also delete stray `HAVE_LIBSSH2` defines.

- Rely on `USE_LIBSSH`, delete parallel `HAVE_LIBSSH_LIBSSH_H`.

  Also delete `LIBSSH_WIN32`, `LIBSSH_LIBRARY` and `HAVE_LIBSSH` from
  `winbuild/MakefileBuild.vc`, these were the result of copy-pasting the
  libssh2 line, and were not having any use.

- Delete unused `HAVE_LIBPSL_H` and `HAVE_LIBPSL`.

Reviewed-by: Daniel Stenberg

Closes #9652

lib/Makefile.m32

@@ -132,7 +132,7 @@ ifneq ($(findstring -rtmp,$(CFG)),)
 endif
 ifneq ($(findstring -ssh2,$(CFG)),)
   LIBSSH2_PATH ?= $(PROOT)/../libssh2
-  CPPFLAGS += -DUSE_LIBSSH2 -DHAVE_LIBSSH2_H
+  CPPFLAGS += -DUSE_LIBSSH2
   CPPFLAGS += -I"$(LIBSSH2_PATH)/include"
   _LDFLAGS += -L"$(LIBSSH2_PATH)/lib"
   _LDFLAGS += -L"$(LIBSSH2_PATH)/win32"
@@ -187,7 +187,7 @@ endif
 ifneq ($(findstring -zlib,$(CFG))$(ZLIB),)
   ZLIB_PATH ?= $(PROOT)/../zlib
   # These CPPFLAGS are also required when compiling the curl tool via 'src'.
-  CPPFLAGS += -DHAVE_LIBZ -DHAVE_ZLIB_H
+  CPPFLAGS += -DHAVE_LIBZ
   CPPFLAGS += -I"$(ZLIB_PATH)"
   _LDFLAGS += -L"$(ZLIB_PATH)"
   _LIBS += -lz