adb: create shell protocol class (take 2).

Adds a new class ShellProtocol to help read and write data with
`adb shell`. This will allow splitting streams and sending out-of-band
data such as exit codes.

Nothing uses the new class yet except the unit tests.

This is the second attempt at this CL, the first is at
http://r.android.com/169600. The problems was using sighandler_t
which is not available on mac. sig_t is used instead which is available
due to _GNU_SOURCE being defined in Android.mk, which causes
_BSD_SOURCE -> __USE_BSD -> sig_t to be defined. Nothing else has been
changed from the original CL.

Bug: http://b/23030641
Change-Id: I7bd7f5a82ad811fbca7a3eee1236d2c55ae57c48