base: Make 'LD_SCRIPT_SO' overwritable

Issue #1048
run: Add 'check_installed' command to run env

'check_installed' takes a command name as argument and tries to call 'which' in
order to find the command path. If that does not succeed, paths like '/sbin' are
'/usr/sbin' searched. On success the absolute path of the command is returned,
on failure 'exit' is called with an error message.

Issue #1048
libc: 'readlink' returns non-zero terminated buffer

Make so to handle zero termination correctly for 'stat' calls and for debugging
output.

Issue #1048
libc_fs: Implemented and test 'unlink' call

Issue #1048
ram_blk: Writable loopback device

Issue #1048
rump: Port of the rump kernels to Genode

For further information see: http://wiki.netbsd.org/rumpkernel/. In this version
I ported the central rump components to Genode in order to take advantage of
NetBSD file system implementation. The new 'dde_rump' repository contains the
Genode version of the rump libraries and a 'rump_fs' server that implements
Genode file-system-session interface. Currently ext2, iso9660, and fat
file-systems are supported.

Issue #1048