net-misc/sstp-client: Amend license, and update libtool .la handling

net-misc/sstp-client: Amend license, and update libtool .la handling

License was incorrectly specified as simply GPL-2, and after consultation
with QA, is now amended to 'GPL-2+-with-openssl-exception'.

Libtool archives are now removed with the 'find -delete' method as
recommended by QA policy guidelines.

Fixed typo in explanatory comment about exampes in src_prepare, and
tidied comments regarding PPPD variables for better readability.

Gentoo-Bug: #603966
Package-Manager: Portage-2.3.6, Repoman-2.3.1