Add the output buffer size in the `unhexify` function, to verify
`olen` doesn't exceed it.

1. Fix indentations.
2. Remove redundant whitespaces.
3. Keep short lines.
4. Grammar fixes.
5. Rephrase function description.

1. Adjust to 80 colums where possible.
2. Add \ remove spaces where needed.
3. Fix alignments.

Fix compilation errors when `MBEDTLS_DTLS_SRTP` not set
1. Add file missed in previous commmit.
2. In sample applications, set `DFL_FORCE_SRTP_PROFILE` to 0.

1. Set correct mki from the `use_srtp` extension.
2. Use mki value received from the client as the mki used by server.
3. Use `mbedtls_ssl_dtls_srtp_set_mki_value()` as a client API only.

Add dtls-srtp to `ssl_client2` and `ssl_server2` examples,
for reference and for allowing in tests.

Signed-off-by: Masashi Honma <masashi.honma@gmail.com>

The previous prototype gave warnings are the strings produced by #cond and
__FILE__ are const, so we shouldn't implicitly cast them to non-const.

While at it modifying most example programs:
- include the header that has the function declaration, so that the definition
  can be checked to match by the compiler
- fix whitespace
- make it work even if PLATFORM_C is not defined:
    - CHECK_PARAMS is not documented as depending on PLATFORM_C and there is
      no reason why it should
    - so, remove the corresponding #if defined in each program...
    - and add missing #defines for mbedtls_exit when needed

The result has been tested (make all test with -Werror) with the following
configurations:

- full with    CHECK_PARAMS with    PLATFORM_C
- full with    CHECK_PARAMS without PLATFORM_C
- full without CHECK_PARAMS without PLATFORM_C
- full without CHECK_PARAMS with    PLATFORM_C

Additionally, it has been manually tested that adding

    mbedtls_aes_init( NULL );

near the normal call to mbedtls_aes_init() in programs/aes/aescrypt2.c has the
expected effect when running the program.

The sample programs require an additional handler function of
mbedtls_param_failed() to handle any failed parameter validation checks enabled
by the MBEDTLS_CHECK_PARAMS config.h option.

Some sample programs access structure fields directly. Making these work is
desirable in the long term, but these are not essential for the core
functionality in non-legacy mode.

Fixes #2170.

This commit adds a command line option `md` to the example application
`programs/x509/cert_req` allowing to specify the hash algorithm to use
when signing the CSR.

And a mis-indented check as well.

This commit replaces multiple `memset()` calls in the example
programs aes/aescrypt2.c and aes/crypt_and_hash.c by calls to
the reliable zeroization function `mbedtls_zeroize()`.

While not a security issue because the code is in the example
programs, it's bad practice and should be fixed.

When using a primality testing function the tolerable error rate depends
on the scheme in question, the required security strength and wether it
is used for key generation or parameter validation. To support all use
cases we need more flexibility than what the old API provides.

If `MBEDTLS_MEMORY_BUFFER_ALLOC_C` is configured and Mbed TLS'
custom buffer allocator is used for calloc() and free(), the
read buffer used by the server example application is allocated
from the buffer allocator, but freed after the buffer allocator
has been destroyed. If memory backtracing is enabled, this leaves
a memory leak in the backtracing structure allocated for the buffer,
as found by valgrind.

Fixes #2069.

* The variables `csr` and `issuer_crt` are initialized but not freed.
* The variable `entropy` is unconditionally freed in the cleanup section
  but there's a conditional jump to that section before its initialization.
  This cmmot Moves it to the other initializations happening before the
  first conditional jump to the cleanup section.

Fixes #1422.