execve: fold {compat_,}do_execve{,at}() into their sole callers

All of them are wrappers for do_execveat_common() and each has
exactly one caller.  The only difference is in the way they are
constructing argv/envp arguments for do_execveat_common() and
that's easy to do with less boilerplate.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>