lua: escape trigraphs in bundled lua sources

Built-in modules are bundled into tarantool in the following way. A lua
file from src/lua or src/box/lua is stored as a string literal in a C
file, then built and linked into tarantool. During startup tarantool
calls luaL_loadbuffer() on this string.

When a Lua source is converted to a C literal, proper escaping is
performed. However there is one case, which was not covered: trigraphs.
The patch adds escaping of question mark symbols to avoid matching ??X
sequences as trigraphs by C preprocessor.

The most simple way to check that it works is to apply the following
patch:

 | diff --git a/src/lua/string.lua b/src/lua/string.lua
 | index 6e12c59ae..2da2dbf4d 100644
 | --- a/src/lua/string.lua
 | +++ b/src/lua/string.lua
 | @@ -425,3 +425,6 @@ string.fromhex    = string_fromhex
 |  string.strip      = string_strip
 |  string.lstrip      = string_lstrip
 |  string.rstrip      = string_rstrip
 | +string.foo = function()
 | +    return '??('
 | +end

And call the function like so:

 | ./src/tarantool -e 'print(string.foo()) os.exit()'

If it printfs `??(`, then everything is okay. If it prints `[`, then
`??(` was preprocessed as the trigraph.

We hit this problem when tried to bundle luarocks-3: it contains
"^(.-)(%??)$" regexp, where `??)` was interpreted as `]`. Debug build or
a build with -DENABLE_WERROR reports an error in the case, but usual
RelWithDebInfo build passes (with -Wtrigraphs warnings) and can show
this unexpected behaviour.

Fixes #4291.

extra/txt2c.c

@@ -70,6 +70,8 @@ int main(int argc, char** argv) {
 		case '\r': fputs("\\r", out); break;
 		case '\\': fputs("\\\\", out); break;
 		case '\"': fputs("\\\"", out); break;
+		/* Don't interpret ??X as a trigraph. */
+		case '?': fputs("\\\?", out); break;
 		default: fputc(c, out); break;
 		}
 	}