Fix: convey enum value signedness into metadata

Currently, passing an enum range of:

  ctf_enum_range("blah", 0, UINT_MAX)

will print a range of 0 ... -1 in the generated CTF metadata, which does
not reflect signedness of the values.

Also, struct ustctl_enum_entry is missing a LTTNG_PACKED attribute,
which is against our protocol rules.

This change needs to be pushed in locked-step into lttng-tools and
lttng-ust, since it breaks the protocol between the two when UST uses
the new enumeration type (introduced in 2.8.0-rc1).

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>