Self-contained busybox - improved stabillity and exec proxying

[Nadav Tasher]

Hi all,

I have made the following changes to the patchset:
1. Disable the use of xvfork on MMU-enabled targets
2. Make NOEXEC depend on MMU-enabled targets (since vfork
expects child to exec)
3. Mimic exec behaviour by closing FD_CLOEXEC marked FDs
and resetting custom signal handlers to SIG_DFL
4. Improved code quality by using smaller functions

Would love to hear your feedback,
Nadav