Support SSL client cert and added ssl_client_cert option

[ggtakec]

Relevant Issue (if applicable)

#2357

Details

There is a request to use a client certificate when connecting s3fs to an S3 server.
This PR is its implementation.

Since s3fs uses libcurl, we have made it possible to specify the following libcurl options:

• CURLOPT_SSLCERT
• CURLOPT_SSLCERTTYPE
• CURLOPT_SSLKEY
• CURLOPT_SSLKEYTYPE
• CURLOPT_KEYPASSWD

The ssl_client_cert option has been added to allow user to specify the libcurl parameters listed above.
Specify the ssl_client_cert option in the following format:

ssl_client_cert=<SSL Cert>[:<Cert Type>[:<Private Key>[:<Key Type>[:<Password>] ] ] ]

• <SSL Cert>
  Specify the SSL client certificate file path.
  If user are using NSS etc., you can specify a Nickname of cert.
• <Cert Type>
  Specify the SSL client certificate type (PEM, P12(for PKCS#12), etc.).
  This value is optional, and if omitted(unspecified or empty string), the PEM type will be used.
• <Private Key>
  Specify SSL private key.
  This value is optional(unspecified or empty string).
  If this value is omitted, the following <Key Type> and <Password> fields are ignored.
• <Key Type>
  Specify the SSL private key type (PEM, P12(for PKCS#12), etc.).
  This value is optional, and if omitted(unspecified or empty string), the PEM type will be used.
• <Password>
  Specify a passphrase to access the SSL private key.
  This value is optional, if omitted(unspecified or empty string), no passphrase will be set.
  However, if user omit this value and the S3FS_SSL_PRIVKEY_PASSWORD environment is set, the value of this will be set as the passphrase (to avoid specifying a passphrase on the command line).

NOTE

This modification has not tested the operation by specifying a client certificate.
(Maybe for that we need to have a client certificate verify function in s3proxy etc.)
Therefore, I believe that this option should remain an unofficial one even if this PR is merged.
I think it would be a good idea to change to the official option while checking its performance.
Finally, it maybe better to specify the passphrase in a file instead of just using options and environment variables.

@gaul
Please let me know your opinion on adding this option.

[wil2business]

@ggtakec
I tested PR code( #2436 ) and fount the solution to work only in foreground mode.
I was not able to mount a directory using a background mode. Also, it did not generate any error message when I attempted to mount us in the background mode.
Thank you for working on this.