Allow TimeoutSec=0 to work as documented in mount units and elsewhere (#6013)

Since commit 36c16a7c ("core: rework unit timeout handling, and add
new setting RuntimeMaxSec=") TimeoutSec=0 in mount units has
cause the mount to timeout immediately instead of never as documented.

There is a similar problem with Socket.TimeoutSec and Swap.TimeoutSec.

These are easily fixed using config_parse_sec_fix_0().

Automount.TimeoutIdleSec looks like it could have the same problem,
but doesn't because the kernel treats '0' as 'no timeout'.
It handle USEC_INFINITY correctly only because that constant has
the value '-1', and when round up, it becomes zero.
To avoid possible confusion, use config_parse_sec_fix_0() as well, and
explicitly handle USEC_INFINITY.