Switch default death test style back to "fast".

Google Test has recently (02/09/2018) switched the default death test
style from "fast" to "threadsafe" in
https://github.com/google/googletest/commit/ec7faa943d7817c81ce7bdf71a21ebc9244dc8de

Threadsafe death tests have been used internally for a while, and are
proven to be a better default.

However, adopting this better default can be challenging for large
projects with a significant investment in custom infrastructure built on
top of Google Test. The same custom infrastructure can make it difficult
for large projects to switch back to the old default by passing in
--gtest_death_test_style=fast.

For the reasons above, the default switch is considered too disruptive,
and this CL reverts it. This CL also introduces the
GTEST_DEFAULT_DEATH_TEST_STYLE preprocesor macro, which replaces the
hard-coded default. The macro can be defined in
gtest/internal/custom/gtest-port.h by projects that are ready to migrate
to thread-safe death tests.