relax valid character restriction in names

[zvr]

[idstring] is a unique string containing letters, numbers, ., - and/or +.

Do we have a reason to not allow a more extensive set of characters?

I propose we also allow _:/#

[goneall]

We use : for parsing document ID for external document references.

Adding _ seems quite reasonable and useful.

We may want to reserve some additional special characters for future use, so I'm a bit more reluctant to add #.

[kestewart]

Request has been made from Yocto group to allow "+" character as well, as it's used in library names.

[goneall]

I believe this is different enough in 3.0 we can close this issue.

@zvr @kestewart if you disagree, please re-open or open a more specific issue.