Document and tidy up one of the more arcane corners of signal

Document and tidy up one of the more arcane corners of signal
handling: why PRE(sys_sigreturn) has to construct a fake syscall
return value which, when written back to the guest state, leaves it
unchanged.  It's only taken me about 3 years to realise why :-)
Fixes to ppc platforms to follow.

git-svn-id: svn://svn.valgrind.org/valgrind/trunk@6650