This patch implements 'execve' in Genode's libc.
The mechanism relies on the dynamic linker's ability to replace the
loaded binary while keeping crucial libraries - in particular the libc -
intact. The state outside the libc is wiped. For this reason, all libc
internal state needed beyond the 'execve' call must be allocated on a
heap separate from the application-owned malloc heap. E.g.,
libc-internal file-descriptor objects must not be allocated or refer to
any memory object allocated from the malloc heap.
Issue #3481
