Dynamic help from agents

[mchitre]

This PR adds support for dynamic help text from agents using a __doc__ parameter in addition to normal help that shell extensions provide. An agent publishing __doc__ must advertise the org.arl.fjage.shell.Services.DOCUMENTATION service. A placeholder @@ in the help text from an agent is replaced with the agent name. This permits writing of help without making assumption on agent name, or having to dynamically generate the help text.

Some changes in how help is displayed:

• Help index is now sorted, so if 2 sources have similar help titles, they appear together.
• If multiple sources have the same help title keyword then the helps are concatenated when shown to the user. This enables generic help for a service and specific help from an agent providing the service to be displayed together.
• Auto generated lists use * as bullet.
• Top-level headings retain # prefix to demarcate sections.

Example:

> help
arp - access to address resolution service
bb - access to baseband service
mac - CSMA MAC
mac - access to medium access control (MAC) service
node - access to node information service
phy - access to physical service
shell - basic shell commands
unet - basic unet commands

> help mac
# mac - access to medium access control (MAC) service

The MAC service provides access to medium access control parameters and functions.
Agents offering the medium access control (MAC) service advise other agents on
when they may be permitted to make transmissions, in an effort to reduce collisions
and improve network throughput.

Examples:
 mac                                            // list MAC parameters
 mac << new ReservationReq(to: 27, duration: 1) // channel reservation request

Parameters:

* mac.channelBusy - indicates if the channel is busy
* mac.reservationPayloadSize - size of payload (bytes) carried in MAC PDU
* mac.ackPayloadSize - size of ack payload (bytes) carried in ACK PDU
* mac.maxReservationDuration - maximum reservation duration
* mac.recommendedReservationDuration - recommended reservation duration

# mac - CSMA MAC

Parameters:

* mac.phy - physical agent used for carrier sensing
* mac.minBackoff - minimum backoff window (seconds)
* mac.maxBackoff - maximum backoff window (seconds)
* mac.reservationsPending - number of reservations in queue (read-only)

[mchitre]

@notthetup I have updated the PR to only look for docs in agents that publish DOCUMENTATION service. This avoids asking and timing out on agents that don't respond to requests (e.g. gateway agents).