envsetup.sh: add functions to enable and generate core dumps

The shell functions in this patch enable crashing processes with the core limit
set correctly to dump core in directory /cores.  They do so by remounting the
root partition, which is RAM-backed, and by creating the 0777-chmodded /cores
under it. They also set the core file pattern in /proc/sys/kernel/core_pattern
to be /cores/core.%p, such that a core dump will have the crashing process' PID
appended to it.  You enable core-dump generation once per boot, as follows:

	coredump-setup

If a process does not have its core-size rlimit set (as most do not), you can
either set it manually by typing "adb shell prlimit <pid> 4 -1 -1", or by
typing coredump-enable <name>, e.g.

	coredump-enable $(pid mediaserver)

Alternatively, you can cause a running process to dump core by sending it a
SIGSEGV via the shell function core <name>, e.g.:

	core $(pid mediaserver)

Change-Id: Ib174e7ee95515fb9866fa6bf0d5b5bf23f3ec61b
Signed-off-by: Iliyan Malchev <malchev@google.com>