Fix reserved identifier violations

The variable "[```__verbose_errno_save```](https://github.com/dermesser/libsocket/commit/b392310#L0R72 "errno saving")" and  preprocessor symbol "[```_MIXED```](https://github.com/dermesser/libsocket/blob/a01f5172b554bfc056f177ba419b3c13a601d3f8/C%2B%2B/Makefile#L18 "Export toggle")" [do not fit](https://www.securecoding.cert.org/confluence/display/seccode/DCL37-C.+Do+not+declare+or+define+a+reserved+identifier "Do not declare or define a reserved identifier") to the expected naming convention of the C language standard.

This update suggestion corresponds to the issue "[reserved identifier violation](https://github.com/dermesser/libsocket/issues/6 "Check your name selection")".