Improving scriptlet related rpm plugin hooks.

Improving scriptlet related rpm plugin hooks.

Three new hooks added:
PLUGINHOOK_SCRIPTLET_PRE_FUNC
Called before scriptlet execution
PLUGINHOOK_SCRIPTLET_FORK_POST_FUNC
Called after fork() but before scriptlet execution
PLUGINHOOK_SCRIPTLET_POST_FUNC
Called after scriptlet execution

Currently pre and post hooks are called for externals and internal lua scripts.
post hook is called even if scriptlet execution has failed and
the return code is given as an argument.

fork_post hook is only called for external scriptlets,
because for internal scriptlets no fork() is currently performed.

Signed-off-by: Panu Matilainen <pmatilai@redhat.com>