base: remove Trace::enabled flag

The DTRACE() macro tests both Trace::enabled and the specific flag. This
change uses the same administrative interface for enabling/disabling
tracing, but masks the SimpleFlags settings directly. This eliminates a
load for every DTRACE() test, e.g. DPRINTF.

base: Do not dereference NULL in CompoundFlag creation

This patch fixes the CompoundFlag constructor, ensuring that it does
not dereference NULL. Doing so has undefined behaviuor, and both clang
and gcc's undefined-behaviour sanitiser was rather unhappy.

misc: Fix a number of unitialised variables and members

Static analysis unearther a bunch of uninitialised variables and
members, and this patch addresses the problem. In all cases these
omissions seem benign in the end, but at least fixing them means less
false positives next time round.

base: explicitly suggest potential use of 'All' debug flags

Without this declaration, new clangs will complain about this value
being unused. It has no explicit use in the codebase, but it can be
useful to turn on all debugging flags while in a debugger to greatly
increase simulator verbosity.

scons: Add warning for missing declarations

This patch enables warnings for missing declarations. To avoid issues
with SWIG-generated code, the warning is only applied to non-SWIG
code.

debug: fix help output

trace: reimplement the DTRACE function so it doesn't use a vector
At the same time, rename the trace flags to debug flags since they
have broader usage than simply tracing. This means that
--trace-flags is now --debug-flags and --trace-help is now --debug-help

debug: create a Debug namespace

debug: Move debug_break into src/base