configure: Add --enable-libssh2 switch and make it independent of Curl

Submitted by Carlos Rafael Giani

Link to original bug (#755989)

Description

Right now, libssh2 autoconfiguration is tied to Curl's. However, libssh2 is a library generic enough (and SSH support a common enough use case) to move it out of the Curl configure block and let it exist independently. Furthermore, it should be possible to explicitely enable/disable it with an --enable-libssh2 switch.