Updated BB_EXECVPE implementation, bb_system implementation

[Nadav Tasher]

Hi!

This third revision to this patch series addresses the following issues:
1. BB_EXECVPE did not restore the environ on failure.
This was solved by creating a copy of environ before clearing it, then
restoring environ from that copy after the BB_EXECVP call.
2. bb_system caused segmentation faults.
This was solved by adding the missing NULL in the argv[] passed to BB_EXECVP.
3. Compilation warnings in tar.
These were solved by using const char* instead of char* when defining gzip_argv.

Appreciate your responses!

Best regards,
Nadav