app: os.setenv() affects os.environ()

os.setenv() and os.environ() are Lua API for

    extern char **environ;
    int setenv();

The Open Group standardized access points for environment
variables. But there is no a word about that environ never
changes. Programs can't relay on that. For example, addition of
a new variable may cause realloc of the whole environ array, and
therefore change of its pointer value. That was exactly the case
in os.environ() - it was using value of environ array remembered
when Tarantool started.

And os.setenv() could realloc the array and turn the saved pointer
into garbage.

Closes #4733

(cherry picked from commit 954d4bdc)

src/lua/env.lua

@@ -9,9 +9,8 @@ ffi.cdef[[
     int   unsetenv(const char *name);
 ]]
 
-local environ = ffi.C.environ
-
 os.environ = function()
+    local environ = ffi.C.environ
     if not environ then
         return nil
     end

test/app/env.result

@@ -24,6 +24,9 @@ type(env_dict)
 ---
 - table
 ...
+err = nil
+---
+...
 test_run:cmd("setopt delimiter ';'")
 ---
 - true
@@ -31,11 +34,46 @@ test_run:cmd("setopt delimiter ';'")
 do
     for k, v in pairs(env_dict) do
         if type(k) ~= 'string' or type(v) ~= 'string' then
-            return false
+            err = {k, v}
+            break
         end
     end
-    return true
-end;
+end
+test_run:cmd("setopt delimiter ''");
 ---
-- true
+...
+err
+---
+- null
+...
+--
+-- gh-4733: os.setenv() should affect os.environ
+--
+size = 0
+---
+...
+for _, __ in pairs(os.environ()) do size = size + 1 end
+---
+...
+for i = 1, size do os.setenv('gh-4733-test-var'..i, tostring(i)) end
+---
+...
+env = os.environ()
+---
+...
+err = nil
+---
+...
+for i = 1, size do                                                  \
+    local value = env['gh-4733-test-var'..i]                        \
+    if value ~= tostring(i) then                                    \
+        err = {i, value}                                            \
+        break                                                       \
+    end                                                             \
+end
+---
+...
+err
+---
+- null
 ...

test/app/env.test.lua

@@ -9,13 +9,32 @@ do os.getenv('location') end
 
 env_dict = os.environ()
 type(env_dict)
+err = nil
 test_run:cmd("setopt delimiter ';'")
 do
     for k, v in pairs(env_dict) do
         if type(k) ~= 'string' or type(v) ~= 'string' then
-            return false
+            err = {k, v}
+            break
         end
     end
-    return true
-end;
-test_run:cmd("setopt delimiter ''")
+end
+test_run:cmd("setopt delimiter ''");
+err
+
+--
+-- gh-4733: os.setenv() should affect os.environ
+--
+size = 0
+for _, __ in pairs(os.environ()) do size = size + 1 end
+for i = 1, size do os.setenv('gh-4733-test-var'..i, tostring(i)) end
+env = os.environ()
+err = nil
+for i = 1, size do                                                  \
+    local value = env['gh-4733-test-var'..i]                        \
+    if value ~= tostring(i) then                                    \
+        err = {i, value}                                            \
+        break                                                       \
+    end                                                             \
+end
+err