Refactoring SSL socket backend implementations

[ming]

There are currently a lot of duplication in the SSL backend implementation, which causes major issues, maintenance difficulties, as well as unnecessary complexity when trying to add a new SSL backend.

The major issues are primarily due to revision differences (one backend (OpenSSL) gets updated/fixed a lot, while the other (GnuTLS) lags way behind). These create behavioral differences, where new features such as the new callback on_accept2() is only available for OpenSSL, and potential security problem, since bug fixes are only applied to one backend, while leaving the others exposed.

Thus refactoring is necessary, to make sure that shared codes are put in a separate file.

[ming]

In 5938:

Fixed #2180: Refactoring SSL socket backend implementations

[ming]

In 5940:

Re #2180: Fixed incorrect early return in pj_ssl_sock_renegotiate()

[ming]

In 5941:

Re #2180: Suppress warning of unreferenced function parameters

[nanang]

In 5951:

Re #2180: Added ssl_sock_imp_common.h/c to PJLIB MSVC2005 project (excluded for build).

[riza]

In 5966:

Re #2180: Added ssl_sock_imp_common.h/c to PJLIB VS2015 project (excluded for build).