Changed BB_EXECVP and BB_EXECVPE implementation, improved code style and squashed patches

[Nadav Tasher]

Hi!

This latest revision contains the following changes:
1. NOEXEC logic was moved to BB_EXECVPE.
BB_EXECVE is now merely a macro to BB_EXECVPE which provides
a default value for envp.
2. Code style was improved to match current code style.
3. Many changes were squashed together to make reviewing
easier.
4. passwd_wrapper implementation was changed.

Thank you all for your replies!

Nadav