: DCCP Miscellany

This is a small set of miscellaneous DCCP patches which initially were very early in the test tree. To simplify going through the patches, they have been deferred until now:

Patch #1: Add socket option to query the current MPS. In the presence of
`EMSGSIZE' complaints and in the absence of fragmentation, this provides support for querying the current Maximum Packet Size.

Patch #2: Consistently sets all 4 different DCCP debug parameters
(dccp_debug, ccid2_debug, tfrc_debug, ccid3_debug) to have type `bool'.

Patch #3: Cures unaligned pointer access. Some non-64 bit quantities are accessed
not on word boundaries, so that the exception handler is called, which is expensive. Using a solution provided by the kernel sources.

Patch #4: Updates API documentation.

Along with the Sync-Flood fixes, these could be considered for general purpose, I'll put them into the test tree for now.