GAP_ExecuteProcess should mimic ExecuteProcess a bit more closely

[fingolfin]

... in order to pass the GAP test suite:

########> Diff in /Users/mhorn/Projekte/GAP/gap.spielwiese/tst/testinstall/kernel/streams.tst:293
# Input is:
ExecuteProcess("","",0,fail,fail);
# Expected output:
Error, ExecuteProcess: <out> must be a small integer (not the value 'fail')
# But found:
Error, MethodError: no method matching GAP_ExecuteProcess(::GapObj, ::GapObj, ::Int64, ::GapObj, ::GapObj)

Closest candidates are:
  GAP_ExecuteProcess(::GapObj, ::GapObj, ::Int64, !Matched::Int64, ::GapObj)
   @ GAP ~/Projekte/OSCAR/GAP.jl/src/exec.jl:3
  GAP_ExecuteProcess(!Matched::String, !Matched::String, ::Int64, !Matched::Int64, !Matched::Vector{String})
   @ GAP ~/Projekte/OSCAR/GAP.jl/src/exec.jl:10