New option company-abort-manual-when-too-short

For manually started completions, backspacing to a prefix shorter
than company-minimum-prefix-length still shows completions.

Setting the new option to t turns that off, so that completions
disappear whenever your prefix gets too short.

With either value of the new option, even if the user changes the
selection, that no longer suffices to make any prefix valid in this
completion invocation.
More robust and simpler logic in prefix test
Test new prefix behavior, option company-abort-manual-when-too-short
Minor tweaks

* Rename --explicit-action to --manual-action.

* Add tests for '("foo" . t), remove some duplicating tests.

* Tweak the defcustom properties.

* Simplify `company--good-prefix-p' a bit more; handle t cdr better.
Update NEWS

Closes #81