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/lua/chartest_runme.lua

@@ -0,0 +1,14 @@
+require("import")   -- the import fn
+import("chartest")   -- import code
+
+function char_assert(char, code)
+  assert(type(char) == 'string')
+  assert(char:len() == 1)
+  assert(char:byte() == code)
+end
+
+char_assert(chartest.GetPrintableChar(), 0x61)
+char_assert(chartest.GetUnprintableChar(), 0x7F)
+
+char_assert(chartest.printable_global_char, 0x61)
+char_assert(chartest.unprintable_global_char, 0x7F)