MTK changes from Per and Lorenzo

MTK changes from Per and Lorenzo
 * NMEA command handler may read back result, ie. error/status handling
 * Timeout during log reading will do a few retries before
  fatal() occurs. Please let me know if this help,
  (especially PPC Mac seems to have problem with large log files)
 * Devices with 4Mbyte memory are handled, fix 2mbyte limit as failover only
 * Erase will check and only enable log if previously enabled.
 * Log reader will check and only enable log if previously enabled.
  !! Let me know if these functions work as expected !!
 * Log bitmask/size changes are handled correct
 * Zero satellites in view issue' is handled
 * Code cleanups from Lorenzo, ...

git-svn-id: http://gpsbabel.googlecode.com/svn/trunk@3255 f51c46e8-681c-474f-0cfe-069cfd0219fb