Build errors in GCC 7.x

[jng]

I tried to build FDO trunk on Ubuntu 18.04 LTS, which includes GCC 7.x but it fails due to the following:

• General: Errors of the form invalid conversion from ‘wchar_t’ to ‘wchar_t*’ in '\0' assignment due to -fpermissive. Fix: Fix up the incorrect assignments.
• General: Various cases of improper null terminator checking error: ISO C++ forbids comparison between pointer and integer [-fpermissive]. Fix: Fix up the incorrect checks.
• MySQL Driver: The <string> header included in stdafx.h now complains about errno due to finding errno.h in the MySQL Driver directory before <cerrno>. Fix: Rather that fight with include path precedence, just rename errno.h to drv_errno.h so <string> will properly resolve errno in <cerrno>
• King Oracle: Providers/KingOracle/src/Provider/c_FdoOra_API3.cpp:104:14: error: cannot convert ‘bool’ to ‘c_MapOraNameToFdoClass::s_data*’ in return. Fix: Return NULL instead of false

[jng]

In 7754:

Fix errors reported by GCC 7 on Ubuntu 18.04, mainly around incorrect NULL terminator assignment and comparison, along with a false-instead-of-NULL return in the King Oracle provider that is no longer valid. Fixes #956