Disable `MBEDTLS_SSL_DTLS_SRTP` by default in the configuration file.

Add a comment that describes that the feature only supportes the
`use_srtp` extension, and not hte full DTLS-SRTP RFC.

Improve readability of the code:
1. move common code to `ssl_internal.h` as `static inline`.
2. Add comments.
3. Use local variables for extension size.
4. Change function signature, by adding buffer size and output length.
5. Take server srtp profile out of the loop.

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.

Signed-off-by: Jeffrey Martin <Jeffrey_Martin@rapid7.com>

Fix errors raised by doxygen.sh test script

Fix script error for identifier check

Make the key material length in mbedtls_ssl_get_dtls_srtp_key_material
to be in\out, like it is done all over the library

Add support mki value in the DTLS-SRTP

1. Add check for prerequisites in check_config.h
2. Add mki value to use_srtp extension
3. address some review comments

As there are some definitions that are defined regardless of
whether MBEDTLS_ECP_RESTARTABLE is defined or not, these definitions
need to be moved outside the MBEDTLS_ECP_ALT guards. This is a simple
move as MBEDTLS_ECP_ALT and MBEDTLS_ECP_RESTARTABLE are mutually
exclusive options.

As there are some definitions that are defined regardless of
whether MBEDTLS_ECP_RESTARTABLE is defined or not, these definitions
need to be moved outside the MBEDTLS_ECP_ALT guards. This is a simple
move as MBEDTLS_ECP_ALT and MBEDTLS_ECP_RESTARTABLE are mutually
exclusive options.

Document when a context must be initialized or not, when it must be
set up or not, and whether it needs a private key or a public key will
do.

The implementation is sometimes more liberal than the documentation,
accepting a non-set-up context as a context that can't perform the
requested information. This preserves backward compatibility.

For mbedtls_pk_parse_key and mbedtls_pk_parse_keyfile, the password is
optional. Clarify what this means: NULL is ok and means no password.
Validate parameters and test accordingly.