Add "random malloc failure" tooling

Still needs to be documented, somehow/somewhere.

The env var OPENSSL_MALLOC_FAILURES controls how often malloc/realloc
should fail.  It's a set of fields separated by semicolons.  Each field
is a count and optional percentage (separated by @) which defaults to 100.
If count is zero then it lasts "forever."  For example: 100;@25 means the
first 100 allocations pass, then the rest have a 25% chance of failing
until the program exits or crashes.

If env var OPENSSL_MALLOC_FD parses as a positive integer, a record
of all malloc "shouldfail" tests is written to that file descriptor.
If a malloc will fail, and OPENSSL_NO_CRYPTO_MDEBUG_BACKTRACE is not set
(platform specific), then a backtrace will be written to the descriptor
when a malloc fails.  This can be useful because a malloc may fail but
not be checked, and problems will only occur later.

Reviewed-by: Richard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/1252)