Bug #357833 Setting RLIMIT_DATA to zero breaks with linux 4.5+

Bug #357833 Setting RLIMIT_DATA to zero breaks with linux 4.5+

We used to set the process datasize rlimit to zero to prevent
any internal use of brk() from having any effect. But later
linux kernels redefine RLIMIT_DATA as the size of any data
areas, including some dynamic mmap memory allocations.

See bug #357833 for the commit that went into linux 4.5
changing the definition of RLIMIT_DATA. So don't mess with
RLIMIT_DATA anymore. Just remember it for use in the syscall
wrappers.

This also cleans up some hacks around the execv and spawn wrappers.

git-svn-id: svn://svn.valgrind.org/valgrind/trunk@15766