scpi_serial: Get rid of intermediate buffer, do not strip newline

scpi_serial: Get rid of intermediate buffer, do not strip newline

Lowlevel access functions should not alter the data. sr_scpi_get_string(),
which is called by most highlevel access functions, strips newlines
in a central place, and is only fed with data which contains newlines
as a final terminator.
IEEE 488.2 definite length blocks may contain arbitrary data, thus the
payload up to the provided length should be passed unaltered.

Track if the last received character is a newline, which can be used
by sr_scpi_get_string() and its callers to determine if the response
is complete.