Provide slightly better cURL debugging instructions for custom.pem files

Summary: Some context in IRC. We can slightly improve users' chances of catching issues here, maybe.

Test Plan: read it; ran commands

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Differential Revision: https://secure.phabricator.com/D8740
Support file attachments on HTTPSFuture

Summary: @zeeg wants this for some build stuff and we'd like it in general to make Mailgun work better. It's a bit messy because of how cURL works. See pages of comments inline.

Test Plan:
  - Added tests for the parser stuff.
  - Made various requests with and without file attachments and saw them encode data as expected.
  - Made some dangerous requests (with "@") and saw them fail.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley, zeeg

Differential Revision: https://secure.phabricator.com/D8831
Censor URIs in service profile log

Summary: Mailgun requests can hit a credentialed URI. Censor credentials from `--trace` URIs.

Test Plan: Ran `bin/mail send-test ... --trace` and saw censored credentials.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Differential Revision: https://secure.phabricator.com/D8833
Latest xhpast for Windows

Summary: This is the latest version of xhpast built from HEAD using MinGW

Test Plan: Ran 'arc liberate' on Windows, it didn't complain about xhpast missing or out of date

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D8843