base: refactor process class (specifically FdMap and friends)

This patch extends the previous patch's alterations around fd_map.  It cleans
up some of the uglier code in the process file and replaces it with a more
concise C++11 version.  As part of the changes, the FdMap class is pulled out
of the Process class and receives its own file.

src/sim/SConscript

@@ -72,6 +72,7 @@ if env['TARGET_ISA'] != 'null':
     SimObject('Process.py')
     Source('faults.cc')
     Source('process.cc')
+    Source('fd_entry.cc')
     Source('pseudo_inst.cc')
     Source('syscall_emul.cc')