A rough cleanup of netd/softap

- Added error code propagation
- Added more elaborate error messages
- Added stricter parameter checking in softap fwreload
- Cleaned up class signatures
- Removed a non-working default AP in setSoftap
- Removed obsolete functions:
  - int addParam(int pos, const char *cmd, const char *arg);
  - int setCommand(char *iface, const char *fname, unsigned buflen=0);
- Removed unused "softap clients" functionality
- Removed unused hostapd communication socket

Change-Id: Iad05069f6547f4b17481991018707d5f9db2500a
Signed-off-by: Alexander Levitskiy <sanek@google.com>