Abolish calls to shell in vold

Never use popen, just execvp directly

Test: Two tests
- Ensure Marlin device boots and vold_prepare_subdirs is called
successfully
- Try adb shell sm set-virtual-disk true, see that eg sgdisk output is
logged.
Bug: 26735063
Bug: 113796163

Change-Id: Icb34140429db85098a0118a2b833772e3620e7ac