lldb: shorten thread names to make logs easier to follow

lldb prepends the thread name to log entries, and the existing thread
name for the FreeBSD ProcessMonitor thread was longer than the kernel's
supported thread name length, and so was truncated. This made logs hard
to read, as the truncated thread name ran into the log message. Shorten
"lldb.process.freebsd.operation" to just "freebsd.op" so that logs are
more readable.

(Upstreaming to lldb still to be done).