dumpstate: improved collection, better error handling.

- Some of the commands used by dumpstate, which drops root, will not collect
anything during a bug report. E.g. df fails on /mnt/secure/asec.
- force flushing of error message
  . without the flush (using gingerbread and not master)
     *** netcfg: Exit code 255
  . with the flush()
     *** exec(netcfg): Permission denied
     *** netcfg: Exit code 255
- add a check before using WEXITSTATUS()

Change-Id: Ia3924b9cc8a12cecab894cbf7f2c79614a358d34
Signed-off-by: JP Abgrall <jpa@google.com>