Generate distinct Smack labels for appId and pkgId

Generate distinct Smack labels for appId and pkgId

Stop hard-wiring Smack labels to "User".
Proper unique labels will be generated:
- "User::App::$appid" for appId
- "User::Pkg::$pkgid" for pkgId

Generated rules are also validated against Smack restrictions for labels.
This imposes the following limitations on appId and pkgId values:
- length up to 244 characters (255 minus prefix length)
- non-printable ASCII characters and space are not allowed
- other prohibited characters: slash, backslash, single quote, double quote

Change-Id: Ib60b1f6fae785919542b2e749bceabbea0b9a89b
Signed-off-by: Rafal Krypa <r.krypa@samsung.com>