TBR: i#95 Linux detach: detaching via dr_app_stop_and_cleanup()

Commit log for first patchset:
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i#95 Linux detach: detaching via dr_app_stop_and_cleanup()

Modeled on Peter Goodman's code at https://codereview.appspot.com/13314047
but with many changes, including:
+ not including any of the re-takeover changes: global var resets, etc.
+ using a futex instead of sleeping and checking a volatile bool.

This does retain the duplication of Windows detach code from the patch.
That will be addressed in an upcoming CL.

Adds a new app API routine dr_app_stop_and_cleanup() which triggers a
detach that can assume it's on an app thread and app stack.

Implements detach cleanup for Linux by splitting the portions that must be
executed by each thread (mainly segment restoration) from the rest of the
cleanup that the detaching thread can perform.

The final go-native step is accomplished via sigreturn using a generalized
dynamorio_futex_wake_and_exit/dynamorio_semaphore_signal_all routine,
renamed to dynamorio_condvar_wake_and_jmp and taking in a target routine to
jump to after the wake.  This target is dynamorio_sigreturn for detach.

Adds two app-detach tests.
---------------

[bruening, 2016-09-13 22:31:22]

Committed as
https://github.com/DynamoRIO/dynamorio/commit/959211659d07f35e356dc9aac0f451f...

Final commit log: 
---------------
i#95 Linux detach: detaching via dr_app_stop_and_cleanup()

Modeled on Peter Goodman's code at https://codereview.appspot.com/13314047
but with many changes, including:
+ not including any of the re-takeover changes: global var resets, etc.
+ using a futex instead of sleeping and checking a volatile bool.

This does retain the duplication of Windows detach code from the patch.
That will be addressed in an upcoming CL.

Adds a new app API routine dr_app_stop_and_cleanup() which triggers a
detach that can assume it's on an app thread and app stack.

Implements detach cleanup for Linux by splitting the portions that must be
executed by each thread (mainly segment restoration) from the rest of the
cleanup that the detaching thread can perform.

The final go-native step is accomplished via sigreturn using a generalized
dynamorio_futex_wake_and_exit/dynamorio_semaphore_signal_all routine,
renamed to dynamorio_condvar_wake_and_jmp and taking in a target routine to
jump to after the wake.  This target is dynamorio_sigreturn for detach.

Adds two app-detach tests.

Review-URL: https://codereview.appspot.com/302710043
---------------