Add OPT_LOAD_MISSING_RULE_EXT
Add $(CWD)/jam to the IncludeModule search path
Update ruleexists to return existence for bound rules, too
Remove UseModule
Handle toolchain=whatever on the command line in addition to TOOLCHAIN=whatever
Implement FindMissingRule in the Jambase
Rename bin/modules/toolchains/c/_helpers/c.jam to bin/modules/c.jam

This will enable any C.??? rules to automatically find and load
bin/modules/c.jam and find the needed rule.
C module: Remove C.SubDirCcFlags, C.SubDirC++Flags, and C.SubDirHdrs
C module: Add C.Flags no-op rule for the workspace build
C module: Add 'optional' option to C.ToolchainHelper
C module: Use _all_ instead of an asterisk when looking for the toolchain spec file
Remove unused line in rule Toolchain in Jambase.jam
JamToWorkspace: Calculate the global defines and headers properly for the workspace

This fixes the Intellisense issues reported by John Brandwood.
C module: Rename C.UseDotNet to C.DotNet

The following renames are performed in this commit:

  * C.UseDotNet -> C.DotNet
  * C.ReferenceDirectories -> C.DotNet.ReferenceDirectories
  * C.StrongName -> C.DotNet
C module: Rename C.UseMFC to C.MFC
C module: Rename rules within the c/midl.jam module

Renames:

  * C.MidlFlags -> C.Midl.Flags
  * C.MidlCompiler -> C.Midl.Compiler
C module: Rename C.UseMySQL to C.MySQL
C module: Rename C.UseOpenSSL to C.OpenSSL
C module: Rename rules within the c/qt.jam module

Renames:

  * C.UseQt -> C.Qt
  * $(QT_FOUND) -> C.Qt.Found
C module: Rename c/win32resource.jam to c/rc.jam

Renames the following rules:

  * C.ResourceCompiler -> C.Rc
  * C.RcFlags -> C.Rc.Flags
  * C.RcDefines -> C.Rc.Defines
  * C.RcIncludeDirectories -> C.Rc.IncludeDirectories