Add support for preloading a shared object containing a dummy execve()

Add support for preloading a shared object containing a dummy execve()
function that just sets error and returns -1.  This adds a
"noexec_file" option to load the filename as well as a "noexec" flag
to enable it unconditionally.  There is also a NOEXEC tag that can
be attached to specific commands and an EXEC tag to disable it.