Refactor flags in method FileSystem::execute_command() (#754684)

Change the two optional boolean parameters into a single optional flags
parameter which uses symbolically defined names.  Makes reading the
execute_command() calls much easier to understand.  (Implemented as bit
field using the same technique as used for Glib::SpawnFlags [1]).

This changes the calls thus:

  execute_command(cmd, od)              -> (cmd, od)
  execute_command(cmd, od, false)       -> (cmd, od, EXEC_NONE)  // [2]
  execute_command(cmd, od, true )       -> (cmd, od, EXEC_CHECK_STATUS)
  execute_command(cmd, od, false, true) -> (cmd, od, EXEC_CANCEL_SAFE)
  execute_command(cmd, od, true , true) ->
                          (cmd, od, EXEC_CHECK_STATUS|EXEC_CANCEL_SAFE)

[1] SpawnFlags bitwise operators in
    /usr/include/glibmm-2.4/glibmm/spawn.h.

[2] False and EXEC_NONE are the default values for the optional third
    parameter before and after this change respectively and both mean
    the same.  This is being used in btrfs::resize() and being kept for
    now despite it being the default.

Bug 754684 - Updates to FileSystem:: and Utils::execute_command()
             functions