exabgp.conf.5: update man page to mostly exabgp4 status.

[he32]

Things to note:

• The 'static' and 'flow' entries have mostly been copied over without too much checking.
• I've tried to update the 'l2vpn' to match the code.
• There are still errors of omissions, ref. the CAVEATS section. The default values for various settings ought to be documented...