vold [1/2]: add ro.vold.umsdirtyratio property

It allows control of what /proc/sys/vm/dirty_ratio is used when USB mounting.
At some platforms like the HTC Desire (bravo) and Samsung Galaxy S family (i9000x) we get a _HUGE_ performance boost when writing to the sdcard when USB mounting if this is set to 20 (which is the default) on the devices above.

It makes it possible to undo or control the effect of this commit:
https://github.com/CyanogenMod/android_system_vold/commit/a28056b38275003895ff5d9576681aca01544822

Signed-off-by: Humberto Borba <humberos@gmail.com>
Change-Id: I86b7bbd02695fb0c19f39448714a219257bf806f
vold [2]: support more than 1 UMS share

Used for devices with both mountable internal and external sdcards.

Original authors:

    Alex Zepeda <alex@inferiorhumanorgans.com>
    Genokolar <genokolar@gmail.com>
    Josh Stone <cuviper@gmail.com>
    mik_os <kiril.mik.os@gmail.com>
    Oran Avraham <oranav@gmail.com>
    Pawit Pornkitprasan <p.pawit@gmail.com>
    Tony Layher <layhertony@gmail.com>

Signed-off-by: Humberto Borba <humberos@gmail.com>
Change-Id: I926ce244255414697373d26d252a9da89aea7226
Merge commit 'refs/changes/73/4873/5' of https://gerrit.omnirom.org/android_system_vold into HEAD
Merge commit 'refs/changes/37/4437/5' of https://gerrit.omnirom.org/android_system_vold into HEAD