[build2] bdep new allows projects names that are invalid c++ namespace names

[Boris Kolpackov]

Klaim - Joël Lamotte <mjklaim at gmail.com> writes:

> Suggestion A:
> 1. allow library name as now (I think it would not be pleasant for users
>     if libraries with dashes in names were not allowed);
> 2. when generating the namespace, change unavailable characters
>     to `_` (underscore) trimmed;
> 3. provide a way to specify the namespace in commands generating code,
> in particular `bdep new` (I don't see such an option).

Fixed as 1-2 but not 3 (it is fairly easy to customize manually so
maybe later). Thanks for the report.