debian: Fill in gecos field for jenkins user

Otherwise when Jenkins runs git, git will complain that the identity is
not set up correctly and will fail.
[JENKINS-19004] set name of remoting threads
plexus-classworlds 2.5.1
Log CannotResolveClassException and similar at FINE, not WARING.
OldDataMonitor already collates these errors and presents them to the user with a fix.
Printing a stack trace for each and every missing class just clutters the log file.
(Especially when restarting after disabling a widely-used plugin.)
Merge pull request #895 from kutzi/JENKINS-19004-channel-executor-name

[JENKINS-19004] set name of remoting threads
[maven-release-plugin] prepare release jenkins-1.528
[maven-release-plugin] prepare for next development iteration
updated changelog for release
updated changelog as a part of the release
merged back the RC branch
avoid log duplication with mvn 3.1 builds
add changelog entry
creating an RC branch
the trunk is toward 1.530-SNAPSHOT
Improved name of Maven project.
[FIXED JENKINS-18846] Update symlinks for Maven module builds.
Need to restore the owner of the describable list so that changes in subsequent sessions will be persisted.
(Issue not reproduced here, but was in similar ArtifactManagerConfiguration: 757fe0844ce0d53d08c9d762ddce4b1190ab8cd4.)
@since
Worth recording a UserInterruption even for the anonymous user.
If JenkinsLocationConfiguration.jenkinsUrl is still null, CoreEnvironmentContributor will silently skip several variables.
No obvious fix for that, but at least document it so it is not so surprising.