Redefine the default boolean type to gmx_bool.

Redefine the default boolean type to gmx_bool.

There is no standard size for 'bool' in C++, so when
we previously defined it to int for C code the data types
(and structs) would have different size depending on your compiler,
both at gromacs build time and when you use the library.
The only way around this is to NOT assume anything about the C++ type,
so we cannot use the name 'bool' in our C code anymore.
The new gmx_bool is of type unsigned char.