elm/quicklaunch: Improve executable path search

elm/quicklaunch: Improve executable path search

Always search executable based on CWD if possible.
This way, elementary_run behaves more like sh where all of
sh script, sh ./script and sh `pwd`/script will work.

Also, remove quicklaunch-specific ELM_MAIN.
This needs some doc, but compiling with -fpie and -pie is much
better.

Note: There's an API/ABI break here, BUT these APIs are marked
as not to be used outside macros. And there isn't any macro
using them :)