Refactor Read_Motd() into Read_TextFile()

Now this function allows to read arbitrary text files into arrays.
IRC_HELP(): Code cleanup
Implement new configuration option "HelpFile"

This new configuration option allows to specify a specially formatted
text file which can be used by the HELP command to provide information
about the commands and their syntaxes.
Document "HelpFile" in sample-ngircd.conf and ngircd.conf.5
Implement Help() function parsing and returning the help text

This function parses the already read in help text and sends the requested
portions to the user. Parsing is done as following when a user user
issues a "HELP <cmd>" command:

 1. Search the file for a line "- <cmd>",
 2. Output all subsequent lines that start with a TAB (ASCII 9) character
    to the client using NOTICE commands, treat lines containing a single "."
    after the TAB as empty lines.
 3. Break at the first line not starting with a TAB character.

This format allows to have information to each command stored in this file
which will not be sent to an IRC user requesting help which enables us to
have additional annotations stored here which further describe the origin,
implementation details, or limits of the specific command.

A special "Intro" block is returned to the user when the HELP command is
used without a command name.
Add doc/Commands.txt which should document all commands

This document can be used as "help text", too, see configuration option
"HelpFile" in ngircd.conf(5).

Please note that this file in its current state is far from complete, only
a few commands are documented, but you should get an idea how it works.

So please send in patches adding the remaining parts! :-)
Add a note that "help file" is updated on startup and REHASH only
Use "${docdir}/Commands.txt" as help text file
Merge branch 'bug145-ProvideHelp'

* bug145-ProvideHelp:
  Use "${docdir}/Commands.txt" as help text file
  Add a note that "help file" is updated on startup and REHASH only
  Add doc/Commands.txt which should document all commands
  Implement Help() function parsing and returning the help text
  Document "HelpFile" in sample-ngircd.conf and ngircd.conf.5
  Implement new configuration option "HelpFile"
  IRC_HELP(): Code cleanup
  Refactor Read_Motd() into Read_TextFile()