Fix to the problem with displaying service intreface with `unknown_t`

[disRecord]

These changes fixes the following problem. If in lua console user tries to display Service interface and one of Operations contains unknown_t as parameter then lua exception occurs.

TaskContext interfaces are displayed properly so I modified service2str accordingly.

[disRecord]

I added getOperationInfo() to TaskContext. getOpInfo() is preserved as alias.

Also note that TaskContext interface still contains getOps() instead of getOperationNames()

[disRecord]

Are there any other comments? This functionality allows to implement lua completion for operations in services: https://github.com/orocos-toolchain/rttlua_completion/pull/6 .

[meyerj]

Looks good to me.

[slavanap]

Few side comments:

• In case Lua compiled without C++ exceptions support, and if luaL_error triggers, destructors for local variables at line #1766 are not called. Thus args variable construction is better to be performed after checks that could lead to luaL_error call.
• lua_pushstring(L, s.c_str()) could be replaced with lua_pushlstring(L, s.c_str(), s.size()) with explicit size specification, because std::string already caches size of its storage, thus one strlen call, performed by lua_pushstring internally, may be eliminated.
• lua_newtable(L); at line #1777 may be replaced with lua_createtable(L, args.size(), 0); (args.size() array elements).
• it's better to replace it++ with ++it at line #1779
• lua_newtable(L); at line #1780 may be replaced with lua_createtable(L, 0, 3); (3 record elements).
• lua_pushstring(L, "name"); lua_pushstring(L, it->name.c_str()); lua_rawset(L, -3); may be replaced with lua_pushlstring(L, it->name.c_str(), it->name.size()); lua_setfield (L, -2, "name");

[disRecord]

Well... These comments concerns not only provided code fragment but the entire file. If we want to preserve the same code style in entire file a major refactoring is needed. I do not think it should be performed in this pull request.

[slavanap]

@disRecord I advise to consider these notes at least in the code chunk you suggested to add. It's up to you and organization members to decide whether introduce them in other places.

[slavanap]

@meyerj Are you going to merge this PR?