[config.cc] Allow group names (identified by @) in the main config file.

These act as macros that are expanded when the config file is read, which is
on startup. Changes to groups membership require a reload of daemon-manager
before they take effect.

Closes: caldwell/daemon-manager#1
[daemon-manager.conf.asciidoc] Document the new @group config syntax.

Closes: caldwell/daemon-manager#1
[config.cc] Make lines in the config file cumulative.

That is:

  david: jim
  david: bob
  @dev: www-data

is now equivalent to:

  david: jim, bob, www-data

(assuming david is in @dev).
[daemon-manager.conf.asciidoc] Typo
[daemon-manager.conf.asciidoc] Add minor heading for the group section.
[daemon-manager.conf.asciidoc] Explain how rules accumulate.