Temporarily relax the dlopen flag.

On MacOSX, with the local flag the 'Module' used by cling is missing quite
a few symbols:

root [0] int a = 2;
root [1] a
ExecutionContext: use of undefined symbol '_ZN5cling16ValuePrinterInfoC1EPN5clang4ExprEPNS1_10ASTContextE'!
ExecutionContext: use of undefined symbol '_ZN5cling20valuePrinterInternal13flushToStreamERN4llvm11raw_ostreamERKSs'!
ExecutionContext: use of undefined symbol '_ZN5cling20valuePrinterInternal18printValue_DefaultEPKvRKNS_16ValuePrinterInfoE'!
ExecutionContext: use of undefined symbol '_ZN5cling20valuePrinterInternal17printType_DefaultERKNS_16ValuePrinterInfoE'!
ExecutionContext::executeFunction: symbol '_ZN5cling16ValuePrinterInfoC1EPN5clang4ExprEPNS1_10ASTContextE' unresolved while linking function '_Z15__cling_Un1Qu31v'!
ExecutionContext::executeFunction: symbol '_ZN5cling20valuePrinterInternal13flushToStreamERN4llvm11raw_ostreamERKSs' unresolved while linking function '_Z15__cling_Un1Qu31v'!
ExecutionContext::executeFunction: symbol '_ZN5cling20valuePrinterInternal17printType_DefaultERKNS_16ValuePrinterInfoE' unresolved while linking function '_Z15__cling_Un1Qu31v'!
ExecutionContext::executeFunction: symbol '_ZN5cling20valuePrinterInternal18printValue_DefaultEPKvRKNS_16ValuePrinterInfoE' unresolved while linking function '_Z15__cling_Un1Qu31v'!
root [2] .q
Add null deref ptr transformer, which instruments the llvm::IR to protect user
code at the prompt accidentally dereferencing a null ptr.

Tests coming soon.
Fix the wrong include filename.
Spelling.
It wasn't Baozeng's fault I committed the old instead of the newest patch.
Add proper initialization.
Replace Decl** with the more accurate Transaction**.

Add output Transaction parameter to Interpreter::parse.
doc update
Fix for deque that was growing during iteration
Factor out module generation.
Move registration code into TModuleGenerator.
Implement header writers.
#include umbrella, not each header separately. Might need Linkdef.h
Also pass Linkdef.h; build sources for pragma parser.
Linkdef files are not passed; fix include, PCM file name.
Silence warning about unused var; will need to add new parameter to RegisterModule().
Reflect new file name.
Fix -D parsing / writing to match what TCling expects.
Do not rely on the declcontext order.
Put the describing headers next to the PCM, delete them afterwards.