Move the definitions for maximum argument and environment size to
Process.h from execve.cpp. This allows sysconf(_SC_ARG_MAX) to return
the actual argument maximum of 128 KiB to userspace.
Not much to say here, this is an implementation of this call that
accesses the actual limit constant that's used by the VirtualFileSystem
class.
As a side note, this is required for my eventual Qt port.
