Remove remaining crypto references

Closes https://github.com/neovim/docs/pull/26

Also added stubs for 'cryptmethod' and 'key', and placeholders for
explanation regarding removal of crypto functionality.
Merge pull request #1563 from Pyrohh/cleanup-docs-encryption

doc: remove remaining crypto references
build: use plainTerminal for tests on Travis CI
Merge pull request #1609 from jszakmeister/use-plain-terminal-for-tests

build: use plainTerminal for tests on Travis CI
Move breakcheck functions from misc1.c to breakcheck.c

Move the functions to reduce the number of os_breakcheck() calls from
misc1.c to a new module breakcheck.

Refactor the code to match neovim style.