libselinux: Refactor rpm_execcon() into a new setexecfilecon()

This new function allows a process to invoke helper programs with
a new execution context based on the filename, this is initially
intended for package managers so that they can easily execute
package scriptlets or maintainer scripts.

Base rpm_execcon() off this new function.

Signed-off-by: Guillem Jover <guillem@debian.org>
Updated libselinux/ChangeLog for next.

Signed-off-by: Stephen Smalley <sds@tycho.nsa.gov>