[kernel] Dispatchers and most kernel objects lockdep instrumented.

- Convert SoloDispatcher to a CRTP template type similar to
PeerDispatcher. This makes the lock classes for different solo
dispatchers distinct.
- Instrument the locks for all dispatchers.
- Instrument the locks for all kernel objects except those that
lock the thread_lock.

Bug: ZX-23
Test: k ut all and system/test/* pass + manual stress testing.

Change-Id: I8106c04911e47ea15ccce8e7abd5fbd62adfa6fe

[socket][share] Always consume the socket_to_share parameter, even on failure

This also fixes a copy/paste bug in the doc for share when the queue
is occupied. The line was copied from the manpage for accept.

Test: core/socket got new tests exercising all the branches here

Change-Id: I3ab7d04eddd133a7659222a0ec04a6e816e3c620

[public][ulib][dev] Add __FALLTHROUGH macro for warning suppression

GCC 7 enables -Wimplicit-fallthrough with -Wextra, so we'll get it.
Clang has it available, so start using it and clean up code accordingly.

The GCC feature will actually recognize some formulaic comments to
suppress the warning. But there's also an explicit syntax, and
C++17 added a standard one. So use that instead, via the new macro
__FALLTHROUGH in <zircon/compiler.h>.

TC-36 #comment Clean up code in preparation for more -Wimplicit-fallthrough

Change-Id: I28309c91c32db1313d52f4255836c5ea687e141f

[kernel][syscalls] Remove redundant filename prefix

All of these files are already in a directory named syscalls.

Change-Id: If4945b107c2ae9787e63dfbfe3750d7dfdba739b