install: verify taskd socket path fits in sun_path

install: verify taskd socket path fits in sun_path

The full path to the taskd socket must fit (including the
terminating '\0') within the sun_path field of the
struct sockaddr_un structure.

Unfortunately, it has a limited length that's much less
than the maximum path length or even the POSIX minimum
maximum path length.

Check during the installation process to make sure that
the full path to the taskd socket will fit in the sun_path
field otherwise complain loudly and abort the installation.

Make the actual length of the sun_path field available in
the `var_sun_path_len` variable.

Signed-off-by: Kyle J. McKay <mackyle@gmail.com>