CI: Switch from Ubuntu 16.04 to 18.04, install libsodium

This commit:

- Switches from Ubuntu 16.04 to 18.04 for all builds, mainly in order to use a more recent version of libsodium.
- Installs libsodium, used by the WireGuard implementation.

.ci/azure-pipelines/linux.yml

@@ -2,9 +2,9 @@ jobs:
 - job: ubuntu_x86_64
   displayName: 'Ubuntu (x86_64)'
   pool:
-    vmImage: ubuntu-16.04
+    vmImage: ubuntu-18.04
   steps:
-  - script: sudo apt-get -y install cmake gcc g++ ninja-build libncurses5-dev libreadline-dev libssl-dev make zlib1g-dev
+  - script: sudo apt update && sudo apt-get -y install cmake gcc g++ ninja-build libncurses5-dev libreadline-dev libsodium-dev libssl-dev make zlib1g-dev
     displayName: 'Prepare environment'
   - script: "$(Build.SourcesDirectory)/.ci/azure-pipelines/linux_build.sh"
     env:

.ci/azure-pipelines/macos.yml

@@ -3,7 +3,7 @@ jobs:
   pool:
     vmImage: macOS-latest
   steps:
-  - script: brew install cmake ninja ncurses readline openssl zlib
+  - script: brew install pkg-config cmake ninja ncurses readline libsodium openssl zlib
     displayName: 'Prepare environment'
   - script: '$(Build.SourcesDirectory)/.ci/azure-pipelines/macos_build.sh'
     env:

.ci/azure-pipelines/windows-steps.yml

@@ -15,7 +15,7 @@ steps:
     path: 'C:/vcpkg/installed'
   displayName: 'Environment storage'
 - script: |
-    vcpkg install openssl zlib --triplet ${{parameters.vcpkgTriplet}}
+    vcpkg install libsodium openssl zlib --triplet ${{parameters.vcpkgTriplet}}
   workingDirectory: C:/vcpkg
   displayName: 'Prepare environment'
 - script: '$(Build.SourcesDirectory)/.ci/azure-pipelines/windows_build.bat'