globalvars.py: new GlobalConstants (gc), GlobalVars (gv) classes

- The attributes of GlobalConstants are non-configurable.  They’re constant
  for a given machine, user, executable and MMGen release.

- The attributes of GlobalVars, `stderr` and `stdout`, are used by the test
  suite to redirect msg() and friends to /dev/null.

- All the aforementioned attributes formerly belonged to GlobalConfig.

scripts/uninstall-mmgen.py

@@ -19,7 +19,7 @@
 import sys,os
 
 import mmgen.opts as opts
-from mmgen.globalvars import g
+from mmgen.globalvars import gc
 from mmgen.util import msg,die
 
 def normalize_path(p):
@@ -53,7 +53,7 @@ opts_data = {
 
 cmd_args = opts.init(opts_data)
 
-if g.platform == 'linux' and os.getenv('USER') != 'root':
+if gc.platform == 'linux' and os.getenv('USER') != 'root':
 	die(1,'This program must be run as root')
 
 if len(cmd_args):