Ignore missing DLL for libcurl

[HebaruSan]

Problem

https://ci.ksp-ckan.space/job/NetKAN/9883/console

Fetching latest ckan.exe

Unhandled Exception:
System.DllNotFoundException: libcurl assembly:<unknown assembly> type:<unknown type> member:(null)
  at (wrapper managed-to-native) CurlSharp.NativeMethods.curl_global_init(int)
  at CurlSharp.Curl.GlobalInit (CurlSharp.CurlInitFlag flags) [0x00001] in <5257a4b91e0a46648a337ec8fa636dea>:0 
  at CKAN.Curl.Init () [0x00017] in <5257a4b91e0a46648a337ec8fa636dea>:0 
  at CKAN.CmdLine.MainClass.Main (System.String[] args) [0x00091] in <5257a4b91e0a46648a337ec8fa636dea>:0 
[ERROR] FATAL UNHANDLED EXCEPTION: System.DllNotFoundException: libcurl assembly:<unknown assembly> type:<unknown type> member:(null)
  at (wrapper managed-to-native) CurlSharp.NativeMethods.curl_global_init(int)
  at CurlSharp.Curl.GlobalInit (CurlSharp.CurlInitFlag flags) [0x00001] in <5257a4b91e0a46648a337ec8fa636dea>:0 
  at CKAN.Curl.Init () [0x00017] in <5257a4b91e0a46648a337ec8fa636dea>:0 
  at CKAN.CmdLine.MainClass.Main (System.String[] args) [0x00091] in <5257a4b91e0a46648a337ec8fa636dea>:0 
Build step 'Execute shell' marked build as failure

The command that's failing is mono ckan.exe version.

Cause

In #3107 we started calling Curl.Init() unconditionally at the start of ckan.exe and netkan.exe to suppress the "non-threadsafe init" message.

Curl.Init() throws that exception if libcurl isn't installed.

Background

https://github.com/KSP-CKAN/CKAN/wiki/libcurl

libcurl is a third party package that CKAN uses. It was required as of CKAN 1.6.6 and made optional in CKAN 1.22.2. Currently it is only used as a fallback if the main downloader fails. In normal use you should be able to get by without libcurl.

We just made it required again. That's no good.

Changes

Now Curl.Init() doesn't care if the DLL isn't found.

[techman83]

Good catch, merging with thanks!