When process_vm_readv fails with EPERM, try PTRACE_PEEKDATA

When process_vm_readv fails with EPERM, try PTRACE_PEEKDATA

process_vm_readv() and ptrace(PTRACE_PEEKDATA) have inconsistent access
control rules wrt traced processes: process_vm_readv() is more likely to
fail with EPERM than ptrace(PTRACE_PEEKDATA) when tracing a process that
has execve'd a privileged executable.

* util.c (umoven, umovestr): If process_vm_readv returned EPERM,
fall back to ptrace(PTRACE_PEEKDATA).

Reported-by: Andrew Guertin <lists@dolphinling.net>