Add support for run-time configuration of the user errors and line en…

[eniv]

This pull request introduces run-time configuration of the line ending and user errors. Run-time configuration is valuable when compiling the library as an independent archive or shared object and linking against it from different applications.

Minor modifications are applied to _scpi_interface_t in order to allow run time configuration, however, legacy compile time configuration is maintained using the following two macros (in scpi_user_config.h):

• USE_RUN_TIME_SCPI_LINE_ENDING
• USE_RUN_TIME_USER_ERROR_LIST

[eniv]

I am not using the new feature for custom registers, but I think it would make sense to allow run time configuration there as well.