serial.c: NetBSD doesn't have OFILL.
Made sr_config_get driver argument a const pointer
Made sr_config_list driver argument a const pointer
Check sr_config_set value argument is not NULL
hantek-dso: Use message logging helpers.
session: Use message logging helpers.
hwdriver: Use message logging helpers.
sr_driver_init(): Improve checks.

Check the relevant arguments for != NULL before calling the actual
driver-specific function, so that the driver can safely assume those
arguments are non-NULL. This removes the need to duplicate these
checks in every driver.

Also, change one SR_ERR to the more correct SR_ERR_MALLOC, and assign
sr_ctx in the rigol-ds1xx2's hw_init() function, like all the other
drivers do.
sr_driver_scan(): Improve checks.

Check the relevant arguments for != NULL before calling the actual
driver-specific function, so that the driver can safely assume those
arguments are non-NULL. This removes the need to duplicate these
checks in every driver.

Also, do some minor whitespace and consistency improvements.
Factor out common hw_init() driver code.

Most drivers do pretty much the same things in their hw_init()
right now, so factor out that code to std_hw_init() in std.c.
Drivers: Explicitly list sr_samplerates fields.

This makes it a bit easier to understand which value means what, just
from looking at the driver code.
device: Cosmetics.
zeroplus: Use message logging helpers.