When debugserver fails to attach to a process on a Darwin

When debugserver fails to attach to a process on a Darwin
system, make a couple of additional checks to see if the
attach was denied via the System Integrity Protection that
is new in Mac OS X 10.11.  If so, return a special E87
error code to indicate this to lldb.

Up in lldb, if we receive the E87 error code, be specific
about why the attach failed.

Also detect the more common case of general attach failure
and print a better error message than "lost connection".

I believe this code will all build on Mac OS X 10.10 systems.
It may not compile or run on earlier versions of the OS.
None of this should build on other non-darwin systems.

llvm-svn: 243511