More predictable behavior for .chown_pid_file(true) when setting group or user.

[hkamphor]

When .chown_pid_file(true) is called with only one of the methods
.group("groupname") or .user("username"), the unset group or user is set to the highest possible gid or uid.

This patch uses the default for the unset user or group. The same behavior as when both were not set.
This results in a more predictable behavior in line with the rest of the code.

Example:

Original code: .chown_pid_file(true) without both .group and .user
629 -rw-r----- 1 root root 7 10 apr 15:26 /tmp/my-daemon.pid

Original code: .chown_pid_file(true) with only .user("nobody")
636 -rw-r----- 1 nobody 4294967294 7 10 apr 15:30 /tmp/my-daemon.pid

With patch: .chown_pid_file(true) with only .user("nobody")
645 -rw-r----- 1 nobody root 7 10 apr 15:32 /tmp/my-daemon.pid