Fix execveat() with AT_FDCWD and relative path, add more checks

Fix execveat() with AT_FDCWD and relative path, add more checks

This update does address two closely related problems:

1) In case execveat() is called with a special file descriptor value
of AT_FDCWD (-100), it should accept this special value, and
interpret the provided pathname as relative to the current working
directory of the calling process (like execve(2)) instead of
failing with EBADF, as it does without this patch.

Covered by LTP testcase execveat01.

https://bugs.kde.org/show_bug.cgi?id=506806

2) Add checks preventing execveat() of symlinked programs in case
AT_SYMLINK_NOFOLLOW was specified.

Add checks preventing execveat() from passing in case invalid
flag was specified.

Covered by LTP testcase execveat02.

https://bugs.kde.org/show_bug.cgi?id=506813