Added "name" to Modules protobuf.

Allow the users to specify a bare library names.

For example, "foo-2.3" will expand to "libfoo-2.3.so" on Linux and
"libfoo-2.3.dylib" on OS X.

Review: https://reviews.apache.org/r/26674
Allowed unloading of a single module.

unloadAll() forces destruction of Owned pointers to the dynamic
library handle. This implicitly causes a dlclose() of all the loaded
module libraries. Unloading a dynamic library can have side effects if
some shared state (e.g., a module) in Mesos points into the library.

Futher more, removed the existing unloadAll() that is no longer
needed.

Review: https://reviews.apache.org/r/26789
Added an advisory message reminding the user to initialize the log.

Review: https://reviews.apache.org/r/26815
Fixed a typo in the last commit.
Supported O_CLOEXEC for os::open on all platforms.

Review: https://reviews.apache.org/r/26715
Added a protobuf::append function.

Review: https://reviews.apache.org/r/26799
Added a protobuf::append test.

Review: https://reviews.apache.org/r/26800
Added the missing --path to the replicated log advisory message.
Fixed verify-reviews.py to not apply submitted reviews.

Review: https://reviews.apache.org/r/26827