patch 8.0.1841: HP-UX does not have setenv()

Problem:    HP-UX does not have setenv().
Solution:   Use vim_setenv(). (John Marriott)
diff --git a/src/misc1.c b/src/misc1.c
index 1ab55f0..76f5025 100644
--- a/src/misc1.c
+++ b/src/misc1.c
@@ -4485,7 +4485,7 @@
 #ifdef HAVE_UNSETENV
     unsetenv((char *)var);
 #else
-    mch_setenv((char *)var, "", 0);
+    vim_setenv(var, (char_u *)"");
 #endif
 }