Add a %C escape sequence for LocalCommand and ControlPath that expands
to a unique identifer based on a has of the tuple of (local host,
remote user, hostname, port).

Helps avoid exceeding sockaddr_un's miserly pathname limits for mux
control paths.

bz#2220, based on patch from mancha1 AT zoho.com; ok markus@
Move failure case text from uvm_pagerealloc() to uvm_pagealloc()

uvm_pagerealloc() is a void function and it doesn't attempt to find
any pages, so it doesn't make sense that it might "return NULL when no
page can be found".
Include <sys/mutex.h> explicitely here.

It will no longer be pulled by uvm_extern.h in the short future.
ok jsg@
escape %C since groff thinks it part of an Rs/Re block;
Revert back to 1.129: pool_init() is called before rwlocks can be
used on some archs.