os/env: use libuv v1.12 getenv/setenv API

- Minimum required libuv is now v1.12
- Because `uv_os_getenv` requires allocating, we must manage a map
  (`envmap` in `env.c`) to maintain the old behavior of `os_getenv` .
- free() map-items after removal. khash.h does not make copies of
  anything, so even its keys must be memory-managed by the caller.

closes #8398
closes #9267

config/CMakeLists.txt

@@ -47,17 +47,8 @@ if(Iconv_FOUND)
   set(HAVE_ICONV 1)
 endif()
 
-check_function_exists(_putenv_s HAVE_PUTENV_S)
-if(WIN32 AND NOT HAVE_PUTENV_S)
-  message(SEND_ERROR "_putenv_s() function not found on your system.")
-endif()
 check_function_exists(opendir HAVE_OPENDIR)
 check_function_exists(readlink HAVE_READLINK)
-check_function_exists(setenv HAVE_SETENV)
-if(UNIX AND NOT HAVE_SETENV)
-  message(SEND_ERROR "setenv() function not found on your system.")
-endif()
-check_function_exists(unsetenv HAVE_UNSETENV)
 check_function_exists(setpgid HAVE_SETPGID)
 check_function_exists(setsid HAVE_SETSID)
 check_function_exists(sigaction HAVE_SIGACTION)

config/config.h.in

@@ -32,8 +32,6 @@
 #cmakedefine HAVE_UV_TRANSLATE_SYS_ERROR
 // TODO: add proper cmake check
 // #define HAVE_SELINUX 1
-#cmakedefine HAVE_SETENV
-#cmakedefine HAVE_UNSETENV
 #cmakedefine HAVE_SETPGID
 #cmakedefine HAVE_SETSID
 #cmakedefine HAVE_SIGACTION