libc rpc: Fix mismatches in prototypes for array bounds.

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.

Reviewed by: imp, emaste
Reported by: GCC -Warray-parameter
Differential Revision: https://reviews.freebsd.org/D36757