* Update htslib and add libcurl variant
This PR updates htslib to version 1.10.2 and adds a libcurl variant. The
libcurl variant defaults to True because, while it is optional, it is
highly recommended by the project developers.
Other things done:
- be consistent with quotes
* Change version in variant description
Apparently, `spack info` does not like the `@` character in a variant
description.
```
==> Error: Incomplete color format: '@' in
@1.3:.
```
ed9ab0668f