This commit renames `lint-enable` into `lint-show-diagnostics`,
makes it hidden, and calls it automatically after diagnostics have
been recovered by `:lint-cleaned-selections`.
The `lint-disable` command becomes `lint-hide-diagnostics`.
The concept of "enabling" diagnostics was inherited from the Clang
support script, but in that case it's not clear why calling `:lint`
should do the work but not render it (similarly to `:spell`).
The `lint-show` command was also renamed into a more descriptive
`lint-show-current-line`.
b4f2c3fe91