Build coremods on Windows, add general support to the cmake script for building modules consisting of multiple compilation units

Instead of special casing m_spanningtree create one project for each subdir in src/modules/ and src/coremods/
Check if Membership::GetPrefixChar() returns 0 before appending it to a string

Spotted by @barosl
Remove User::WriteTo() functions; use User::WriteFrom() instead

The removed functions were rarely used and all they did was to call User::WriteFrom() with swapped arguments. On call sites, WriteTo() hardly offers any readability improvement over WriteFrom().
Move User::SendAll() into core_privmsg

This functionality is only used by the PRIVMSG and NOTICE handlers
Add support for generating DANE TLSA records to genssl.