iproto: replace iproto_constant with string arrays

There are no substantial gaps in the remaining IPROTO constant enums so
there's no need in iproto_constant struct. Instead we can generate
string arrays, as we usually do. This is more flexible because it allows
us to look up a name by code. It's also consistent with iproto_type and
iproto_key names.

The only tricky part here is the iproto_flag enum because it contains
bit masks. To generate names for the flags, we add the auxiliary enum
iproto_flag_bit that contains bit numbers.

Follow-up #8443
Follow-up commit b3fb883b ("iproto: export IPROTO constants to Lua
automatically")

NO_DOC=refactoring
NO_TEST=refactoring
NO_CHANGELOG=refactoring