Fix warning about unused result of write() in priv::callback_base::stop()

On systems with a strict compiler, the buildprocess will complain:
  [ 70%] Building CXX object src/CMakeFiles/conky.dir/update-cb.cc.o
  conky/src/update-cb.cc: In member function ‘void conky::priv::callback_base::stop()’:
  conky/src/update-cb.cc:49:34: warning: ignoring return value of ‘ssize_t write(int, const void*, size_t)’, declared with attribute warn_unused_result [-Wunused-result

This patch fixes this warning (and by doing this also checks if the write worked)
Fix warning about unused result in strerror_r() redefinition

On systems, the buildprocess will complain:
  [  7%] Building CXX object src/CMakeFiles/conky.dir/c++wrap.cc.o
  conky/src/c++wrap.cc: In function ‘std::string strerror_r(int)’:
  conky/src/c++wrap.cc:66:37: warning: ignoring return value of ‘char* strerror_r(int, char*, size_t)’, declared with attribute warn_unused_result [-Wunused-result]

According to "man 3 strerror_r" the buffer used as 2nd arg is not always used, this patch also fixes that problem.