Fix a memory leak when kern_execve() fails fatally.

Fix a memory leak when kern_execve() fails fatally.

The callers of execve() are taking care of releasing the argument
buffer for exec.  However, there might be situations where exec
ran into a dead end and can't come out again because it already
has destroyed the calling process image.  In these cases kern_execve
was calling exit1() to destroy the current process.  This however means
that the argument buffers were never freed, thus causing the leak.

In combination with the recent change to using the objcache with a
hard upper allocation limit, this meant that at some point no binaries
could be execed anymore, stalling callers of sys_exec() in objcache_get.

To fix this business, kern_execve() now does not call exit1() on
leathal errors anymore, but instead returns -1 (in contrast to valid
errno values which are >= 0) to indicate that the current process has
to commit suicide after cleaning up used resources.