base: refactor process class (specifically FdMap and friends)

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.