This commit applies the documentation improvements noticed and applied
while adding the documentation for the new X.509 CRT verification API
mbedtls_x509_crt_verify_with_cb() to the existing verification APIs.

So far, there were the following CRT verification functions:
- `mbedtls_x509_crt_verify()` -- no profile, no restartable ECC
- `mbedtls_x509_crt_verify_with_profile()` -- profile, no restartable ECC
- `mbedtls_x509_crt_verify_restartable()` -- profile, restartable ECC
all publicly declared and offering increasing functionality.

On the implementation-side,
- `mbedtls_x509_crt_verify()` resolves to
  a call to `mbedtls_x509_crt_verify_with_profile()` setting
  the profile to `NULL`, and
- `mbedtls_x509_crt_verify_with_profile()`
  resolves to a call to ``mbedtls_x509_crt_verify_restartable()`
  setting the ECC restart context to NULL.

This commit adds two more functions to this zoo:
- `mbedtls_x509_crt_verify_with_cb()`
- `x509_crt_verify_restartable_cb()`

Here, `mbedtls_x509_crt_verify_with_cb()` is similar to
`mbedtls_x509_crt_verify_with_profile()` but uses a CA callback
instead of a static CA list, and no restart context.

`x509_crt_verify_restartable_cb()` is similar to
`mbedtls_x509_crt_verify_restartable()` but allows to either use
a static list of trusted CAs _or_ a trusted CA callback.

On the implementation-side,
- the body of `mbedtls_x509_crt_verify_restartable()` is moved to
  `x509_crt_verify_restartable_cb()`, and the new version of
  `mbedtls_x509_crt_verify_restartable()` just resolves to
  `x509_crt_verify_restartable_cb()` with the trusted CA callback
  set to NULL.
- The new function `mbedtls_x509_crt_verify_with_cb()`
  forward to `x509_crt_verify_restartable_cb()` with the restart
  context set to `NULL`.

There's no change to the implementation yet, and in particular,
`mbedtls_x509_crt_verify_with_cb()` isn't yet usable.

The CA callback changes introduce mbedtls_calloc() and
mbedtls_free() to ssl_client2 and ssl_server2, which
wasn't defined unless MBEDTLS_PLATFORM_C was set.

Change the naming to reflect that the function uses a new ca callback
feature to distinguish different callbacks.