lang/mono: update to version 5.2.0.215 (and enhance USES=mono)

Highlights:

• New Roslyn compiler for C# available
• Improved support for nuget packages in USES=mono

General:

• fix pkg-plist: mono now produces '.pdb' debug files instead of '.mdb'
• bump all dependant ports

USES=mono:

• properly handle caching of nuget packages
• add support for multiple feeds for nuget packages
• add support for nuget dependencies in a separate file
• add support for paket packages

lang/mono:

• update to version 5.2.0.215
• automate certificate initialisation [2]
• increase test coverage
• mark as conflicting with net/czmq (conflicting on makecert) [1]
• patch mono to use $PREFIX/share/mono instead of /usr/share/.mono

devel/google-gdata:

• use nunit.framework nuget package as the Mono shipped version is no longer suppport.
• switch to using csc(1) for compiling (mcs(1) is depreciated).
• use delayed signing (and then sign with sn(1)) as csc(1) does not support signing.
• fix reference to system assemblies (the '.dll' suffix is required).
• fix reference to HttpUtility: csc(1) is more strict about scoping

devel/monodevelop:

• reroll distinfo (no changes to content)

lang/fsharp:

• reroll distinfo (no changes to content)

security/gnome-keyring-sharp:

• delay sign (then sign with sn(1)) as csc(1) does not support direct signing.

PR: 223188 [1]
PR: 209670 [2]
Differential Revision: https://reviews.freebsd.org/D13752