lua: push characters as unformatted 1-character strings

Since Lua 5.3 the "%c" format character in lua_pushfstring will produce
the string "<\XXX>" (XXX being a decimal code sequence) when
given unprintable characters.

Use lua_pushlstring instead to reproduce the old behavior.

Examples/test-suite/chartest.i

@@ -0,0 +1,15 @@
+%module chartest
+
+%inline %{
+char printable_global_char = 'a';
+char unprintable_global_char = 0x7F;
+
+char GetPrintableChar() {
+  return 'a';
+}
+
+char GetUnprintableChar() {
+  return 0x7F;
+}
+
+%}