Add support for sieve protocol

[tjikkun]

This is not in upstream nginx (yet), but is available at
https://github.com/AntagonistHQ/nginx/tree/sieve_v2

Pull Request (PR) description

This PR allows the word "sieve" to be used as a protocol. Upstream nginx doesn't support this, so I can understand if this PR gets denied for that. On the other hand, it is a very minor change so I am still hoping it gets accepted.

[puppet-community-rangefinder]

nginx::resource::mailhost is a type

that may have no external impact to Forge modules.

This module is declared in 9 of 578 indexed public Puppetfiles.

---

These results were generated with Rangefinder, a tool that helps predict the downstream impact of breaking changes to elements used in Puppet modules. You can run this on the command line to get a full report.

Exact matches are those that we can positively identify via namespace and the declaring modules' metadata. Non-namespaced items, such as Puppet 3.x functions, will always be reported as near matches only.

[b4ldr]

Upstream nginx doesn't support this, so I can understand if this PR gets denied for that. On the other hand, it is a very minor change so I am still hoping it gets accepted.

i'd probably allow it as it is really minor but looping in some of the more seasoned voxpupuli contributors for the ok

cc @bastelfreak @ekohl