scpi: Synchronize read, write and write+read operations.

scpi: Synchronize read, write and write+read operations.

This ensures that SCPI read/write/write+read operations are thread safe.
F.e.: If a write operation expects a return value (in other words: a
read operation), it is not allowed to be interrupted by another write
operation.

To simplify things, the SCPI helper functions are moved from
scpi/helpers.c to scpi/scpi.c and also are renamed to fit the naming
scheme.

libgpib in particular will abort the program execution in case of
concurrent operations.