Add log message if LLProcessLauncher child fails to execv().

On a Posix platform (vfork()/execv() implementation), if for any reason the
execv() failed (e.g. executable not on PATH), the viewer would never know, nor
the user: the vfork() child produced no output, and terminated with rc 0! Add
logging, make child terminate with nonzero rc.
Remove pointless addArgument(const char*) overload: this does nothing for you
that the compiler won't do implicitly.
In llupdateinstaller.cpp, remove pointless c_str() call in addArgument() arg:
we were starting with a std::string, then extracting its c_str(), only to
construct a whole new std::string from it!