Introduced 'Accepted' responses for libprocess messages.

Review: https://reviews.apache.org/r/20276
Introduced 'Libprocess-From' to identify a libprocess "message".

Review: https://reviews.apache.org/r/20277
Add os::exists(pid_t pid) to test if pid is running.

Uses kill(0, pid) to check pid validity. This is much cheaper than using
os::process(pid) which constructs a full Process object.

Review: https://reviews.apache.org/r/20970
Update process::reap to use os::exists(pid).

Review: https://reviews.apache.org/r/20971
Move OOM log message to appear only on actual OOM

Review: https://reviews.apache.org/r/20139
Change cgroups::assign to assign all threads.

Use cgroups.proc (rather than tasks) so all threads in the pid's
threadgroup are assigned.

Documentation/cgroups/cgroups.txt:
"You can use the cgroup.procs file instead of the tasks file to move all
threads in a threadgroup at once. "

Added a test to verify all threads are assigned.

This obviates the recently introduced assignAllThreads and thus it has
been removed.

Review: https://reviews.apache.org/r/19951
Fixed comment length in cgroups tests.

This style issue started creeping up in other RR's Review bot feed back:

Error:
 Checking 426 files using filter --filter=-,+build/class,
+build/deprecated,+build/endif_comment,+runtime/vlog,
+whitespace/comma,+whitespace/ending_newline,+whitespace/forcolon,
+whitespace/indent,+whitespace/line_length,+whitespace/tab,
+whitespace/todo
src/tests/cgroups_tests.cpp:829:  Lines should be <= 80 characters
long  [whitespace/line_length] [2]
Total errors found: 1

Review: https://reviews.apache.org/r/21098
Extend the Linux launcher to use clone.

The launcher can now support clone'ing processes with namespace flags.

Review: https://reviews.apache.org/r/20816
Added DynamicLibrary to stout.

Addition of DynamicLibrary class to stout, to enable future plugins
inside of mesos.

Review: https://reviews.apache.org/r/21013
Fix for mesos fetcher not applying the executor bit.

Review: https://reviews.apache.org/r/21127
Updated tests to use a strict registry, by default.

Review: https://reviews.apache.org/r/20820
Enabled unbundled usage of http-parser.

Updates libprocess configure check for http-parser to build against
system dependencies outlined in MESOS-1071 + fixes libprocess local
'make check' which was broken.

Review: https://reviews.apache.org/r/20983
Adds Singularity to Mesos Frameworks page.
Cleaned up the logging level configuration.

Review: https://reviews.apache.org/r/20850
Documented Clock::settle.
MESOS-1310: Changed 'reply' to 'send' in slave registration.

Review: https://reviews.apache.org/r/21183
Refactored ZooKeeperImpl into ZooKeeperProcess.

See MESOS-1318 for more details.

Review: https://reviews.apache.org/r/21182
Fixed a bug with containerization in MesosTest.

Review: https://reviews.apache.org/r/21180
Added a test to ensure a recovered slave that does not re-register is removed.

Review: https://reviews.apache.org/r/21176
Added a test that ensure a recovered slave is not removed.

Review: https://reviews.apache.org/r/21186