This is a huge patch that should be a global NOP.
For unix type platforms it enables -Wconversion to issue warnings (-> error)
for all automatic casts that seem less than ideal but are normally concealed
by the toolchain.
This is things like passing an int to a size_t argument. Once enabled, I
went through all args on my default build (which build most things) and
tried to make the removed default cast explicit.
With that approach it neither change nor bloat the code, since it compiles
to whatever it was doing before, just with the casts made explicit... in a
few cases I changed some length args from int to size_t but largely left
the causes alone.
From now on, new code that is relying on less than ideal casting
will complain and nudge me to improve it by warnings.
Mbedtls mbedtls_x509_crt_parse() returns 0 for success which is good.
But it has a complicated idea about what to return on fail... if it
couldn't make even one cert from the data, then it returns a negative
return indicating the parsing problem and there is nothing to free.
If it managed to parse at least one cert, instead it retuns a positive
number indicating the number of certs it didn't parse successfully,
and there is something to free.
Adapt the code to understand this quirk.
