drivers/main.c: allow to specify a user for each driver via ups.conf

[jimklimov]

Closes: #1288

Enhances earlier ability to start ALL drivers as a non-built-in account, by setting user = "name" in ups.conf global section, adding a similar ability to override this further at each driver's level.

Tested to accept and report the flag (along with debug-level passthrough from upsdrvctl, done by #1287), e.g.:

# ./drivers/upsdrvctl -DDDd start nutdev1
Network UPS Tools - UPS driver controller 2.7.4-4660-gc31748b
   0.000000     [D1] Starting UPS: nutdev1
   0.000859     [D2] 1 remaining attempts
   0.001456     [D2] exec:  /usr/local/ups/bin/usbhid-ups -DDD -a nutdev1
Network UPS Tools - Generic HID driver 0.45 (2.7.4-4660-gc31748b)
USB communication driver (libusb 1.0) 0.42
   0.000000     [D3] main_arg: var='driver' val='usbhid-ups'
   0.000673     [D3] main_arg: var='port' val='auto'
...
   0.005066     [D3] main_arg: var='user' val='root'
   0.005440     [D1] Overriding previously specified user 'nobody' with 'root' specified for driver section
...

Anticipated as most-useful when counter-acting non-persistent device filesystem node permissions, so local-media drivers (USB, maybe serial).

Additionally, this fixes a possible issue with earlier global-level user setting, where the filesystem socket could be created by the driver as not accessible to unprivileged upsd process. Now this should behave better by default, and can be twisted explicitly with new group setting in global and/or driver level configuration.

[jimklimov]

Testing with a user = ... specified at both global level and in the driver section:

   0.000000     [D1] Overriding previously specified user 'nobody' with 'default' specified in global section
...
   0.005517     [D1] Overriding previously specified user 'default' with 'root' specified for driver section
...
...

And for command-line option having precedence (though apparently the order of processing differs from my expectations when writing the message texts):

# ./drivers/upsdrvctl -DDDd -u www-data start nutdev1
...
   0.000000     [D1] Overriding previously specified user 'nobody' with 'default' specified in global section
...
   0.005441     [D1] Overriding previously specified user 'default' with 'root' specified for driver section
   0.005598     [D1] Built-in default user for drivers 'root' was ignored due to 'www-data' specified on command line
...