[New feature] satisfy the https private key decrypt requirement

[helinbo2015]

1. feature background

As a https server, for better security consideration, user may not keep the private key(for example: named as tls.key) in a clear text. it may use some encryption algo to encrypt the tls.key for protecting from server inbreak.
so when before use tls.key to create a https connection, user first decrypt the tls.key.

2. current implementation

now, in the evhtp_ssl_init(), cfg->privfile is used directly. this means that user should use a unencrypted privfile. user can not use a encrypted privfile.

    SSL_CTX_use_PrivateKey_file(htp->ssl_ctx,
                                cfg->privfile ? cfg->privfile : cfg->pemfile, SSL_FILETYPE_PEM);

3. solution

i'd like add a member callback (for example: decrypt_privfile_cb) in the evhtp_ssl_cfg_t struct. if this callback is set, it will be called to decrypt the privfile and used by SSL_CTX_use_PrivateKey().

@NathanFrench if this feature is welcome, i can make a commit. Thanks in Advance.

[NathanFrench]

This is actually a really good idea. Take the security out of the libraries hands.

I welcome this type of ingenuity.

[helinbo2015]

@NathanFrench Please review the commit. any suggestions are welcome.
thank you in advance.
BTW: i have tested the customize_decrpyt_privfile_cb in my machine and it works ok.

[NathanFrench]

Merged, thank you very much.