add preservation of comment lines

[wififreedom]

This pull request implements preservation of comments in configuration files.

Comment lines before an entry, and the comment at the end of an entry are considered to be associated with that entry.

Example:

# this comment line is associated with section2

  # this comment line is associated with section2
config type section2 # this comment is associated with section 2
# this comment line is associated with opt
    
        # this comment line is associated with opt
        option opt 'val' # this comment is associated with opt
    
      # this comment line is not associated with any entry

Wih option -D, uci works as before and strips comments. The following commands have enhanced behaviour:

• uci import preserves comments of unchanged configuration entries, and imports entries with their associated comments
• uci export exports entries with their associated comments
• uci 'commit preserves comments of unchanged configuration entries when committing changes with uci commands 'add', 'delete', 'set', 'add_list', and/or 'del_list'.
• uci get '<config>.<section>' '#' shows the section comment
• uci get '<config>.<section>.<option> '#' shows the option comment
• uci set '<config>.<section>=<value>' '<comment>' sets the section value and comment
• uci set '<config>.<section>.option=<value>' '<comment>' sets the option value and comment
• uci add_list '<config>.<section>.option=<value> '<comment>' adds a list entry with value and comment
• uci changes also shows comments
• uci show does not show comments
• uci batch supports the above commands

Features:

• Comments are auto-indented if preserved, set or added.
• The new delta file format is both backward compatible and forward compatible. Older uci versions interpret the addition in the new format as comment and ignores it.
• Comments at the end of the file are not preserved because they do not belong to an entry.

I've added lots of shunit2 tests to verify the implementation.

An example of a result: the above example after and option 'opt2' has been added:

# this comment line is associated with section2
# this comment line is associated with section2
config type section2 # this comment is associated with section 2
        # this comment line is associated with opt
        # this comment line is associated with opt
        option opt 'val' # this comment is associated with opt
        # comment line for new option 'opt2'
        option opt2 'val2'

[GeorgeSapkin]

You'll need to look into the submission guidelines regarding commit formatting, etc.