Validate asec names.

Make sure asec names only contain alphanumeric, underscores,
dots, or dashes. Don't allow double dots.

Bug: 12504045
Change-Id: I3dd0350c79327dc91a5a10d5724d85d99814e769
Merge branch 'cm-11.0' of github.com:CyanogenMod/android_system_vold into cm-11.0