exabgp.1: update man page based on 4.2.21's --help output.

[he32]

Points for discussion:

• I mention both the new and previous syslog facility. It could be argued that the old should not be mentioned.
• It is not entirely clear to me what the actual difference is between -test and -validate (and the wording isn't super precise about that).
• It appears the exabgp.api.highres setting is gone (no longer in --help output, anyway).
• The -folder argument appears to be gone (also no longer in --help output).
• The list of standards has not been reviewed or updated here, I am sure the list has expanded over time, a few drafts have become RFCs or exist in newer versions. Input solicited.