gnu: vsftpd: Use CentOS version and patches.

* gnu/packages/ftp.scm (vftpd)[source]: Use CentOS source RPM.
[arguments]: Adapt the 'unpack phase, and apply CentOS patches in a new
'apply-CentOS-patches phase.
[inputs]: Add openssl, linux-pam, and libcap.
[native-inputs]: Add p7zip and cpio.

Signed-off-by: Tobias Geerinckx-Rice <me@tobias.gr>