Add support for line continuations (a.k.a. newline escapes) to config file parser

[mbuesch]

Status Quo

Currently a key=value pair in the configuration files letmein.conf and letmeind.conf cannot span multiple lines.

Suggested enhancement

Allow line continuation by backslash-escaping of newlines.
Example:

[FOO]
bar=baz\
  biz

shall be equal to

[FOO]
bar=baz  biz

Care must be taken to not interpret continuation lines starting with # as comments.

[FOO]
bar=baz\
#  biz

shall be equal to

[FOO]
bar=baz#  biz

Pros

Long lines can be broken down and be made more readable. This could be especially useful for the [RESOURCES] section.

Cons

It is not fully compatible with existing configurations.
An existing configuration might use a literal backslash at the end of the line:

[NFTABLES]
exe = /foo/bar\

This would currently mean exe is equal to the string /foo/bar\.

However, this probably is only a theoretical problem.

Open questions

• Should we support backslash-escaping at the end to support literal backslashes: bar=biz\\ would yield the value biz\