Revert "lat_syscall: switch from getppid -> getpid"

This reverts commit 70bfa20902bc1c59c283b83b47bc140c3a9a3da4.

Using getpid gets useless results since glibc caches (!) the
getpid results... Fall back to getppid.
lat_syscall: add gettimeofday test

This should be useful to measure vdso effects and not suffer from
the getpid effect of libc caching.
bw_mem: Improve readability
script: wire gettimeofday test