use consistent scheme for chosen MAC addresses

Take a similar MAC address scheme within run scripts like
within the nic bridge to simplify DHCP configuration.
rump: Stabilize rump libs and fs server

* Perform sanity check before calculating memory available to rump kernel
* Distinguish between 'Hard_context' and 'Hard_context_thread', so no dead
  threads (that will not be started) are created
* Install signal-handler for memory-resource requests. This way the fs server
  will not block forever when the quota is execeeded and a resource requests
  fails, leaving the rump kernels to perform necessary actions

Fixes #1127
terminal_session: Add label argument to connection
noux: Handle SYSCALL_KILL in switch (fix warning)
noux: Decouple Dir_file_system from file systems

By introducing the new 'File_system_registry', we can remove the
knowledge about the actual file-system implementations from the
'Dir_file_system'. Thereby the code becomes more generic, which is
a precondition for using it as the basis for Genode's VFS library.

Issue #999
libc_noux: style fix
test/libc_ffat: Operate on cwd instead of root dir

The test used to rely on a writable root directory. To reuse the test
for the new VFS, where '/' is never writable, I changed all absolute
paths to relative paths.
tar_fs: Return correct size of stat'ed dirs

Genode's file system interface returns the number of directories
multiplied by sizeof(Directory_entry) as size of a directory. The
tar_fs server used to return zero. The fix counts the sub nodes of
the given directory.
file system: Exception for non-permitted symlinks

Until now, there was not exception type for the condition where a
symlink was created on a file system w/o supporting symlinks, e.g., FAT.
The corresponding file-system server (ffat_fs) used to return a negative
handle as a work around. I added 'Permission_denied' to the list of
exceptions thrown by 'File_system::Session::symlink' to handle this case
in a clean way.
mixer: Increase stack size of entrypoint
Move VFS from noux to 'os/include/vfs'

By separating the VFS from Noux, we become able to reuse the
VFS for libc-using programs. The most substantial change is the
removal of Noux::Sysio data structures from the VFS. Instead, the
Noux::Sysio refers to the VFS types now.

The new VFS library is located at 'os/include/vfs/'.

Furthermore, the patch removes various code duplications related
to pseudo file systems that provide a single file (block, zero,
random, terminal, null). The new 'Single_file_system' holds the
common boilerplate code for those.

Issue #999
vfs: LOG file system
vfs: ROM file system
vfs: inline file system
libc: Add fd status and flags to File_descriptor

Those values need to be stored local to the FD. Including
them in the 'File_descriptor' eases the handling of libc
plugins.
libc: Make File_descriptor::fd_path char const *
libc: Add 'rmdir' to plugin interface
libc_terminal: Use prio to override libc_vfs
libc_fs: Use priority higher than libc_vfs

This way, we make sure that libc_fs is used if both plugins are
present, which happens during the transition to libc_vfs.
fs test: Appropriately use rmdir instead of unlink