build: Don't add version extra string to the version number.

build: Don't add version extra string to the version number.

If this string was non-empty, then it broke a lot of things because
autoconf and other parts of the build system were expecting this
string to contain a simple MAJOR.MINOR.RELEASE version number.

This requires changes to guestfish and guestmount so they use the
guestfs_version API to fetch the version from the library.  (The
Perl tools were already doing it this way).  In a way this is more
accurate, because it's no longer hard-coded in the binary, but
fetched from the dynamically linked libguestfs.so.