fix(conf) overcome penlight's 'list_delim' setting

[thibaultcha]

Penlight's parsing would infer some settings as tables because of their
format, and our config parser would then contain tables values instead
of strings, which would make tests such as value == "on" obsolete and
would not correctly infer our booleans.

• add list_delim placeholder for config parsing
• move trailing comment removal earlier in the check_and_infer function
• add tests for dnsmasq = off and custom_plugins file parsing

[Tieske]

lgtm

[WALL-E]

from_file_conf, err = pl_config.read(s, {
  smart = false,
  list_delim = "_blank_" -- mandatory but we want to ignore it
})

_blank_: Is only used to ignore this feature(list_delim), should not be used in the configuration file

Do I understand this correctly? @thibaultcha

[Tieske]

@WALL-E yes, the parser requires that value in its options, but it should never be used.