nss: only read logging config from environment variables

nss: only read logging config from environment variables

log_parse_environment() uses should_parse_proc_cmdline() to determine whether
it should parse settings from the kernel command line. But the checks that
should_parse_proc_cmdline() apply to the whole process, and we could get a positive
answer also when log_parse_environment() was called from one of the nss modules.
In case of nss-modules, we don't want to look at the kernel command line.

log_parse_environment_variables() that only looks at the environment variables
is split out and used in the nss modules.

Fixes #22020.