Implement the FIXME that ends an user action with a semicolon

Implement the FIXME that ends an user action with a semicolon
if it seems necessary.

* src/scan-code.l (flex rules section): Flag cpp directive from
any `#' to the first unescaped end-of-line.  Semicolon is not
needed after `;', `{', '}', or cpp directives and is needed after
any other token (whitespaces and comments have no effect).
* tests/actions.at (Fix user actions without a trailing semicolon):
New test.
* tests/input.at (AT_CHECK_UNUSED_VALUES): Add semicolons to
to make user actions complete statements.
Adjust column numbers in error messages.
* tests/regression.at (Fix user actions without a trailing semicolon):
Remove.  Covered by new test.