[System] Don't overwrite custom Connection header on HttpWebRequest
[System] Add crude (for now) socket stealing of an HttpWebRequest.

HttpWebRequest is a very tricky piece of code when it comes to its socket, this patch is more like a hack to get something working out of it but it's definitely not the best option.
[System] Implement a first pass of ClientWebSocket.
Merge pull request #717 from mono/client_websockets_impl

Add implementation of ClientWebSocket
[mkbundle] New mkbundle options for better handling integration with native libs.

New --skip-scan flag allow us to handle resource assemblies as opaque data.
New --static-ctor flag allows us to better interop with weird C libraries requirements.
Bump mono to 3.2.5
Add a couple of casts to scan_thread_data () to shut GCC up.

This would result in the following warnings on e.g. PowerPC:

sgen-gc.c: In function 'scan_thread_data':
sgen-gc.c:4045:6: warning: passing argument 1 of 'conservatively_pin_objects_from' from incompatible pointer type [enabled by default]
sgen-gc.c:1467:1: note: expected 'void **' but argument is of type 'void * (*)[32]'
sgen-gc.c:4045:6: warning: passing argument 2 of 'conservatively_pin_objects_from' from incompatible pointer type [enabled by default]
sgen-gc.c:1467:1: note: expected 'void **' but argument is of type 'void * (*)[32]'