Handle multi-receiver NOTICE and PRIVMSG

This changes znc to handle multi-receiver messages in the same way it handles multi-channel join or parts.  This will ensure that no more than one target is ever given to a module for any given call.
Fix valueless message tags (closes #684)
Introduce CaseSensitivity enum class

The enum is a bit more verbose, but leads to more readable code:

str.Equals("foo", true)
// vs.
str.Equals("foo", CString::CaseSensitive)

Deprecate the old Equals() and leave out the length parameter
from the new version => use StartsWith() or StrCmp() instead.
CString::Starts/EndsWith(): allow specifying case sensitivity
Escape message tag values (ref #684)
Add CString::Find() with case sensitivity support
Add CString::Contains() for convenience

This is more convenient and readable than comparing Find() to npos,
which is a common task:

$ git grep "find(" | grep "npos" | wc -l
49
Fix #680 - make channels follow user-level settings when appropriate
Merge pull request #686 from jpnurmi/string

CString improvements
Merge pull request #689 from NuclearW/multitarget-notice+privmsg

Handle multi-receiver NOTICE and PRIVMSG
Merge pull request #681 from jpnurmi/setuservars

Fix #680 - make channels follow user-level settings when appropriate
Merge pull request #693 from jpnurmi/tags

Valueless & escaped message tags
[log] $WINDOW should be last because it could contain %

Thanks to @NuclearW for spotting
Update Csocket to 71dac61

Csocket has received a bunch of const correctness fixes. Notice that
the signature of one virtual method, Csock::ConvertAddress(), has
changed => CZNCSock has been updated accordingly.
Merge pull request #696 from jpnurmi/csocket

Update Csocket to 71dac61
cleanup superfluous semi-colons
Remove redundant CWebSocket::GetModule()

CWebSocket inherits CSocket::GetModule()
Fix GetClients() const correctness

It’s dangerous to give a non-const reference to an internal
container that the API users are not supposed to modify.
const correctness fixes
Update Grammar on Modules section of ZNC.