Avoid future ABI breakage by hardcoding enum values.

Avoid future ABI breakage by hardcoding enum values.

Whenever we added a field to the enums in libsigrok.h, we were running
the risk of breaking ABI compatibility. Any reordering of existing values
or the addition of entries other than at the bottom of the list would
cause an ABI breakage.

By hardcoding the values we ensure that any added field will have an
unused value, and will not take a value previously used by a different
flag. By doing this, we avoid confusing frontends compiled with an
earlier API.

We use 10000 as the first entry of each enum, and each "category" gets
an explicit number (10000, 20000, 30000, and so on).

Also avoid making the first value of an enum zero. A value of zero is
used as terminator item in some lists.

Remove explicit "DUMMY" (terminator) enum entries, they're not needed.

Signed-off-by: Alexandru Gagniuc <redacted>