LibCore: Add Core::Process::spawn()

This is a simple wrapper around posix_spawn() that will help us simplify
a bunch of very verbose posix_spawn() invocations.

This first version only supports the simplest case: executing an
executable without passing arguments or doing anything fancy. More
features can be added to cover more cases. :^)

Userland/Libraries/LibCore/CMakeLists.txt

@@ -21,6 +21,7 @@ set(SOURCES
     NetworkResponse.cpp
     Notifier.cpp
     Object.cpp
+    Process.cpp
     ProcessStatisticsReader.cpp
     Property.cpp
     Socket.cpp