Making JSON-RPC accept more content types

According to the RFC for JSON-RPC over HTTP at
http://www.jsonrpc.org/historical/json-rpc-over-http.html
"Content-Type SHOULD be 'application/json-rpc' but MAY be
'application/json' or 'application/jsonrequest'"

This patch fixes HTTPJsonRpcHandler to allow for any of those three
content types when accepting JSON-RPC HTTP requests.

This also fixes compatibility with the popular Python library jsonrpclib
which uses the application/json-rpc content type.
[Settings] Silence the analyzer about a (not) possible null pointer dereference.
[Settings] Initialize members in constructors.
[Settings] Remove unused variables.
[Settings] Operator=' should return &.
[Settings] Function parameter '...' should be passed by reference.
[Settings] Prefer prefix ++/-- operators for non-primitive types.
[Settings] Possible inefficient checking for 'settingMap' emptiness.
[Settings] Make functions static if possible.
[GUIControlSettings] Make GetID() const.
[AdvancedSettings] Removed unused member variable and moved m_initialized = true at the end of Initialize().
FIX: add libm to librtmp
Merge pull request #2830 from koying/fixdroidlibrtmp

FIX: add libm to librtmp
Merge pull request #2872 from tecknicaltom/master

webserver: allow JSON-RPC to accept more content types
Merge pull request #2688 from ace20022/cleanup_settings

[Settings] Clean up things found by Cppcheck