Set TZ to avoid repeated stat("/etc/localtime")

With glibc, each time the strftime() function is used (twice per process in a typical ps -fe run), a stat("/etc/localtime") system call is used to determine the timezone. Not only does this add extra system call overhead, but when multiple ps processes are trying to access this file (or multiple glibc programs using strftime) in parallel, this can trigger significant lock contention within the OS kernel.

Since ps is not intended to run for long periods of time as a daemon (during which the system timezone could be altered and PS might reasonably be expected to adapt its output), there is no benefit to repeatedly doing this stat(). To stop this behavior, explicitly set the TZ variable to its default value (:/etc/localtime) whenever it is unset. glibc will then cache the stat() result.