libc rpc: Fix mismatches in prototypes for array bounds.

[jhb]

Summary

Various RPC functions used a bare pointer in function prototypes to
describe fixed-length buffer arguments but used a fixed-length array
in the function definition. The manual page for these functions
describes the parameters as being fixed-length buffers, so update
the prototypes to match the definitions.

Reported by: GCC -Warray-parameter

[imp]

This even works with c89 standard, so we're good there.