Add install-libssh2.sh

Add a utilty script to install the libsh2 library under /usr/local/lib.

Instructions copied from PowerPMAC_EPICS_Developers_Manual.pdf,
and converted into a shell script.

Some extensions had been made to run under Mac OS X and Linux systems
with APT (like Debian) or YUM (like Scientific Linux).
Merge pull request #2 from tboegi/install_libssh2

Add install-libssh2.sh