Drop _C from option names

[gilles-peskine-arm]

The fact that certain options are equivalent to enabling the content of one .c file is an implementation detail. Furthermore, for an increasing number of options called MBEDTLS_XXX_C, this is no longer true. For example, MBEDTLS_RSA_C and MBEDTLS_ECP_C are roughly two files; MBEDTLS_SSL_CLI_C/MBEDTLS_SSL_TLS_C/MBEDTLS_SSL_SRV_C are each multiple files; etc.

We should rename all options to just convey “I want this feature”. For example, MBEDTLS_SSL_CLI (or to be more modern MBEDTLS_TLS_CLIENT? MBEDTLS_WANT_TLS_CLIENT?). And remove “internal” options like MBEDTLS_SSL_TLS_C.

Related: https://github.com/Mbed-TLS/mbedtls/issues/8107