patch 9.0.2070: [security] disallow setting env in restricted mode

Problem:  [security] disallow setting env in restricted mode
Solution: Setting environment variables in restricted mode could
          potentially be used to execute shell commands. Disallow this.

restricted mode: disable allow setting of environment variables

Setting environment variables in restricted mode, may have some unwanted
consequences. So, for example by setting $GCONV_PATH in restricted mode
and then calling the iconv() function, one may be able to execute some
unwanted payload, because the `iconv_open()` function internally uses
the `$GCONV_PATH` variable to find its conversion data.

So let's disable setting environment variables, even so this is no
complete protection, since we are not clearing the existing environment.
I tried a few ways but wasn't successful :(

One could also argue to disable the iconv() function completely in
restricted mode, but who knows what other API functions can be
influenced by setting some other unrelated environment variables.
So let's leave it as it is currently.

closes: #13394
See: https://huntr.com/bounties/b0a2eda1-459c-4e36-98e6-0cc7d7faccfe/

Signed-off-by: Christian Brabandt <cb@256bit.org>

runtime/doc/starting.txt

@@ -1,4 +1,4 @@
-*starting.txt*  For Vim version 9.0.  Last change: 2023 Oct 17
+*starting.txt*  For Vim version 9.0.  Last change: 2023 Oct 20
 
 
 		  VIM REFERENCE MANUAL    by Bram Moolenaar
@@ -249,10 +249,10 @@ a slash.  Thus "-R" means recovery and "-/R" readonly.
 					*-Z* *restricted-mode* *E145* *E981*
 -Z		Restricted mode.  All commands that make use of an external
 		shell are disabled.  This includes suspending with CTRL-Z,
-		":sh", filtering, the system() function, backtick expansion
+		":sh", filtering, the |system()| function, backtick expansion
 		and libcall().
-		Also disallowed are delete(), rename(), mkdir(), job_start(),
-		etc.
+		Also disallowed are |delete()|, |rename()|, |mkdir()|,
+		|job_start()|, |setenv()| etc.
 		Interfaces, such as Python, Ruby and Lua, are also disabled,
 		since they could be used to execute shell commands.  Perl uses
 		the Safe module.

src/evalfunc.c

@@ -9723,6 +9723,12 @@ f_setenv(typval_T *argvars, typval_T *rettv UNUSED)
     if (in_vim9script() && check_for_string_arg(argvars, 0) == FAIL)
 	return;
 
+    // seting an environment variable may be dangerous, e.g. you could
+    // setenv GCONV_PATH=/tmp and then have iconv() unexpectedly call
+    // a shell command using some shared library:
+    if (check_restricted() || check_secure())
+	return;
+
     name = tv_get_string_buf(&argvars[0], namebuf);
     if (argvars[1].v_type == VAR_SPECIAL
 				      && argvars[1].vval.v_number == VVAL_NULL)

src/version.c

@@ -704,6 +704,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    2070,
 /**/
     2069,
 /**/