selinux: a new module implementing "restorecon" functionality

selinux: a new module implementing "restorecon" functionality

* src/selinux.c: A new module implementing "restorecon" functionality.
There are two main functions to adjust the type of the referenced
file system item.  defaultcon() will setup the process context so
that new items will have the required context without races.  This is
the preferred method.  For existing files, the equivalent restorecon()
is available which has two modes.  With the "local" parameter set to
false, restorecon() will adjust the type according to the system
configuration for that file, and set to true will update the context
as per the context for the current process (disregarding type).
* src/selinux.h: Likewise.
* po/POTFILES.in: Reference the new module.