cleanup environment variable handling + unit tests

* removed a putenv() implementation which isn't needed anymore
  * mch_getenv() and mch_setenv() are now functions in src/os/env.c
  * removes direct calls to getenv() and setenv() outside of src/os/env.c
  * refactored the logic of get_env_name into mch_getenvname_at_index
  * added unittests for the functions in os/env.c

src/os/env.c

@@ -0,0 +1,65 @@
+/* vi:set ts=2 sts=2 sw=2:
+ *
+ * VIM - Vi IMproved	by Bram Moolenaar
+ *
+ * Do ":help uganda"  in Vim to read copying and usage conditions.
+ * Do ":help credits" in Vim to see a list of people who contributed.
+ * See README.txt for an overview of the Vim source code.
+ */
+
+/*
+ * env.c -- environment variable access
+ */
+
+#include <uv.h>
+
+#include "os.h"
+#include "../misc2.h"
+
+#ifdef HAVE_CRT_EXTERNS_H
+#include <crt_externs.h>
+#endif
+
+const char *mch_getenv(const char *name)
+{
+  return getenv(name);
+}
+
+int mch_setenv(const char *name, const char *value, int overwrite)
+{
+  return setenv(name, value, overwrite);
+}
+
+char *mch_getenvname_at_index(size_t index)
+{
+# if defined(AMIGA) || defined(__MRC__) || defined(__SC__)
+  /*
+   * No environ[] on the Amiga and on the Mac (using MPW).
+   */
+  return NULL;
+# else
+# if defined(HAVE__NSGETENVIRON)
+  char **environ = *_NSGetEnviron();
+# elif !defined(__WIN32__)
+  /* Borland C++ 5.2 has this in a header file. */
+  extern char         **environ;
+# endif
+  // check if index is inside the environ array
+  for (size_t i = 0; i < index; i++) {
+    if (environ[i] == NULL) {
+      return NULL;
+    }
+  }
+  char *str = environ[index];
+  if (str == NULL) {
+    return NULL;
+  }
+  int namesize = 0;
+  while (str[namesize] != '=' && str[namesize] != NUL) {
+    namesize++;
+  }
+  char *name = (char *)vim_strnsave((char_u *)str, namesize);
+  return name;
+# endif
+}
+