wallet: wire -sendto into main(), where it never was (#119)

CmdSendTo() has been in walletcmd.cpp since #115 and nothing declared
it or called it. The flag was dead: `bitflash -sendto=ADDRESS,AMOUNT`
started a node and said nothing, because an unrecognised argument is
not an error here.

The v1.2.14 notes announce it under "A headless node can finally
spend", and the headless Linux binary is exactly the one that does not
have it -- `-help` there lists /newaddress and stops. The published
Windows binary does have it, because it was built from a working tree
that carried these two files uncommitted. So the artifacts disagree
with each other and neither matches the tag.

This adds only what was missing: the declaration in walletcmd.h, the
dispatch in main(), and the -help line. No behaviour change to
CmdSendTo itself.

Checked on a build of this branch, against a throwaway data directory:

  -help                          lists /sendto=ADDRESS,AMOUNT
  -sendto=NOTANADDRESS,1         "Not a valid address", exit 1
  -sendto=<valid>,-5             "Not a valid amount", exit 1
  -sendto=<valid>,999            refused on an empty wallet, exit 1

All five self-tests still pass.

0 files changed, 0 insertions, 0 deletions