Migrating from C style to C++ style of coding in nest plugin

Migrating from C style to C++ style of coding in nest plugin

Replacing fgets/fputs with fstream equivalents and using std::string
in nest_plugin, wherever applicable.

Change-Id: I5c54556876cc09d1816672a1c30d641856b1c1c7
Signed-off-by: vijendrx <vijendrax.kumar@intel.com>
Reviewed-on: https://gerrit.iotivity.org/gerrit/17579
Reviewed-by: Joseph Morrow <joseph.l.morrow@intel.com>
Tested-by: jenkins-iotivity <jenkins@iotivity.org>
Reviewed-by: Todd Malsbary <todd.malsbary@intel.com>
(cherry picked from commit f3016c9416863700ab1c161b104ac46005d07296)
Reviewed-on: https://gerrit.iotivity.org/gerrit/19169
Reviewed-by: Uze Choi <uzchoi@samsung.com>