TRACE: make sb-debug:arg available to all options.

Not all options which evaluated forms had sb-debug:arg working.

Fixes lp#1357826.
Solve GC issues caused by interrupt handling.

Neither of the alloc_ functions in alloc.c are GC safe, which could be
fixed on x86oids, but is a bit more harder on precise GC platforms.
The most often used function, alloc_sap, can actually be removed,
because the sap can be stack allocated, since it's used for handling
interrupts.
sb-bsd-sockets: disable IPv6 tests on win32.
IPv6 is not implemented on windows.

Closes lp#1386761.
Improve MAKE-LOAD-FORM-SAVING-SLOTS on STRUCTURE-OBJECT.

- use simpler code than SLOT-SETTER-LAMBDA-FORM.
- use a trick based on :SB-JUST-DUMP-IT-NORMALLY if it can be
  shown to have identical behavior to a user's MAKE-LOAD-FORM.
  This occurs when and only when the user's code is
  MAKE-LOAD-FORM-SAVING-SLOTS for all slots.

A consequence of the second point is that the first only matters
when a :SLOT-NAMES argument excludes some slots. Creating large trees
of constant data, such as needed by cl-protobufs, by setting a single
variable to a structure computed through macros expanding to
(DEFVAR *A-BIG-SCHEMA* #S(SCHEMA ....)) now compiles in a fraction of
the time it used to. [The downloadable version of cl-protobufs
uses STANDARD-OBJECT, making this patch inapplicable though]

A test comprised of one DEFVAR initialized to '#.(COMPUTE-THINGS)
where COMPUTE-THINGS returned a list of 10,000 instances of a
structure of 6 slots was sped up thusly:
 Test              |  Fasl size |  Compile   |  Load
 ------------------|------------|------------|--------
 baseline          |  17,504 KB |  83 sec    | .26 sec
 simplified sexprs |  16,311 KB |  45 sec    | .17 sec
 FOP-based dumping |   1,858 KB |   0.26 sec | .02 sec
Don't remove safety checks for SLOT-VALUE-USING-CLASS.

SLOT-VALUE-USING-CLASS may be called with a slot definition from a
different class, which is actually sometimes useful, but it may be for
a slot that doesn't exist. And the code for optimized SVUC used
(safety 0), avoiding array bounds checks, causing very hard to track
down issues.
Just remove the safety declaration.
Remove distracting "It would be better ..." comment.

119d1c157e519573074720b7897a9fa918329ac5 did that thing.
Use STD-COMPUTE-SLOTS for {STRUCTURE,CONDITION}-CLASS. Fixes lp#1049423