agilent-dmm: Fix SEGV during incomplete reply from meter.

If buf contains exactly the string "Agilent Technologies" (for example if
there are bugs in the timeout logic or serial driver causing the reply from
the meter to be abridged) then this code will SEGV. This is because tokens[1]
is NULL but only tokens[2] and tokens[3] (both of which are undefined) are
NULL checked.

Can be trivially corrected by NULL checking tokens[1] as well.

Signed-off-by: Daniel Thompson <daniel@redfelineninja.org.uk>
fx2lafw: Organize driver into api.c / protocol.[ch].

This now matches the naming conventions of the other drivers.
output/analog: add revolutions per minute unit
add cyrustek es519xx generic protocol parser
add ISO-TECH IDM103N serial-dmm driver
es519xx: Cosmetics, coding style, minor fixes.
es519xx: Fix incorrect packet size, and a typo.