Centrally handle environment properties and prepare support for multi-kernel mode side-by-side to Linux.

* Introduce an "environment" module to centrally determine and provide information about the environment.
  This currently includes unikernel vs. multi-kernel, whether uhyve is used, and whether proxy is used.
  It also centrally parses the command-line.

* Move all SyscallInterface implementations along with their proxy/uhyve-specific functions to a subfolder
  "librs/syscalls/interfaces".
  Rename SingleKernel to Generic to disambiguate it from other single-kernel modes.

* Fix running HermitCore Rust in unikernel mode when network initialization failed or "proxy" is not used.

* Handle multi-kernel mode wherever required.
  We currently lack init_mmnif_netif() on the lwIP side to initialize the mmnif network interface for multi-kernel mode.

* Add a kernel_message_buffer module compatible to the C implementation.
  Don't initialize a serial port and output all kernel messages to this buffer when multi-kernel mode is detected.

* Add information about the "vga" feature to Cargo.toml to make it actually possible to build with VGA support.
  This is still disabled by default.