Stop using the same value for LD_NO_MOCK and LD_MESG
LD_NO_MOCK and LD_MESG have the same value, which is a bug in 0.4.1.1-alpha and later.
We'll need to make log_domain_mask_t into a u64, and change the values of the domain flags so they go down from 63 (not 31).
We'll also need to change the corresponding type in Rust to u64. Maybe we should define a type, rather than using u64 everywhere.
This is a bug on #28226 (moved), which is Sponsor 31 must, so I'm marking this ticket as sponsor 31 must.