Revert "Disable "--pre""

This reverts commit 1cf87111c0dc4e1ee77a40253b4c8a7b49fc4efb.
Remove unneeded signal mask in connection thread

Threads inherit the signal mask from the parent thread. There is no need
to set the signal mask again.

A new test ensures that build will fail if this does not work as
documented on some platform.
Revert "Revert "Disable "--pre"""

This reverts commit 9901544a126979e611825af361547e3c49419553.
Update changelog.
Increase timeouts a bit to cope with Travis slowness. This should make anything slower as the runner (TestProcess) will kill the helper process at the end of the context.
Add the docs env to Travis matrix.