Syscalls: Take an i64 instead of a u64 for the gas limit

[Stebalien]

Currently, send:

1. Takes a u64 as the gas limit, but we're effectively limited to i64.
2. Treats 0 as "all the gas".

Proposal:

1. Take an i64 (not particularly controversial).
2. Some of:
   1. Treat any negative value as "all the gas".
   2. Treat any negative value as "invalid".
   3. Treat 0 as 0 instead of all the gas.

We can punt this till 2.2, I think?

[Stebalien]

We now treat u64::MAX as "all the gas".