MINOR: tcp/http/conf: extends the keyword registration options

MINOR: tcp/http/conf: extends the keyword registration options

This patch permits to register a new keyword with the keyword "tcp-request content"
'tcp-request connection", tcp-response content", http-request" and "http-response"
which is identified only by matching the start of the keyword.

for example, we register the keyword "set-var" with the option "match_pfx"
and the configuration keyword "set-var(var_name)" matchs this entry.