Remove length limit on PKINIT PKCS#12 prompt

Long pathnames can trigger the 128-byte prompt length limit in
pkinit_get_certs_pkcs12.  Use asprintf instead of snprintf.  Also
check the result of the prompter invocation.

ticket: 8011
target_version: 1.13.1
tags: pullup
Add custom error formatting (err_fmt config param)

    Add [libdefaults] err_fmt, a parameter for additional formatting of
    error messages.  %C gets substituted with the numeric error code.
    %M gets substituted with the original error message.

    For example:

        [libdefaults]
                err_fmt = %M (http://docs.foo.example/krb5/errors/%C)

    will produce errors like:

        Cannot find KDC for realm "FOO.EXAMPLE" (http://docs.foo.example/krb5/errors/-1765328230)
Include file ccache name in error messages