Support v2 fstab format

Auto detect and support both the v1 and v2 fstab formats
Support putting TWRP style flags in a separate /etc/twrp.flags file

twrp.flags format is the same as twrp.fstab (v1 with TWRP flags)

Support using a wildcard in a block device and find all partitions:
/usb-otg vfat /dev/block/sda*

Support using sysfs entries (voldmanaged) and read uevents and scan for
wildcard partitions from uevent data. (twvold?)

May not be complete for some of the newer flags found in fstabs in newer
build trees and there is a slim chance of a crash if the user removes a
removable device while TWRP is performing actions. May need to add some
kind of mutex to prevent the 2 threads from causing this crash. We need
to start somewhere though and this change is pretty innocuous when not
using a v2 fstab.

Change-Id: I617d97c7db332cbe671a9d2b8ad98b3d9c4f03cc