Add runtime configuration logging upon `USR2` signal.

[mpaperno]

As per suggestion from @realsimix in #36, logs key=value pairs of runtime values for all configurable SpamPD options upon USR2 signal to parent process. E.g.

sudo kill -USR2 `cat /run/spampd.pid`
--------
spampd[78805]: SpamPD v2.612 [Perl 5.28.1, Net::Server::PreFork 2.009, SA 3.4.2, rules v1891891]
spampd[78805]: [running config start]
spampd[78805]: auto-whitelist=(undefined)
spampd[78805]: child-name-template=%base_name: child #%child_count(%child_status) [req %req_count/%req_max, time lst/avg/ttl %(req_time_last).4f/%(req_time_avg).4f/%(req_time_ttl).4f, ham/spm %req_ham/%req_spam, rules v%sa_rls_ver %ns_typ_acr]
spampd[78805]: childtimeout=360
spampd[78805]: debug=(undefined)
spampd[78805]: detach=0
spampd[78805]: dose=0
spampd[78805]: group=131 131
spampd[78805]: homedir=/var/cache/spampd
spampd[78805]: host=127.0.0.1
spampd[78805]: local-only=1
spampd[78805]: log-rules-hit=1
spampd[78805]: logfacility=mail
spampd[78805]: logfile=syslog
spampd[78805]: logident=spampd
spampd[78805]: logsock=(undefined)
spampd[78805]: max-servers=5
spampd[78805]: max-spare=4
spampd[78805]: maxrequests=30
spampd[78805]: maxsize=64
spampd[78805]: min-servers=1
spampd[78805]: min-spare=1
spampd[78805]: pid=/run/spampd.pid
spampd[78805]: port=10025
spampd[78805]: relayhost=127.0.0.1
spampd[78805]: relayport=10026
spampd[78805]: relaysocket=(undefined)
spampd[78805]: saconfig=/etc/spampd.conf
spampd[78805]: satimeout=285
spampd[78805]: set-envelope-from=0
spampd[78805]: set-envelope-headers=1
spampd[78805]: setsid=0
spampd[78805]: socket=(undefined)
spampd[78805]: socket-perms=(undefined)
spampd[78805]: tagall=1
spampd[78805]: user=124
spampd[78805]: [running config end]