mraa.c: added user check into mraa_init() & increase verbosity

mraa_init() will now:
- print user name and EUID of the process instead of UID,
- print full platform text name in addition to numeric type,
- return, log and print an error if started by non-root user

Fixes #64

Signed-off-by: Alex Tereschenko <alext.mkrs@gmail.com>
Signed-off-by: Brendan Le Foll <brendan.le.foll@intel.com>
mraa.c: remove DEBUG define

debug DEFINE crept in from 6bc2b0fb53865c85c786f7f2a0393d88e7784c08

Signed-off-by: Brendan Le Foll <brendan.le.foll@intel.com>
mraa.c: Fixed potential segfault on free()-ing NULL in mraa_deinit()

Signed-off-by: Alex Tereschenko <alext.mkrs@gmail.com>
Signed-off-by: Brendan Le Foll <brendan.le.foll@intel.com>