[Darwin] Do not crash (failed assertion) after PT_KILL ptrace error

[Darwin] Do not crash (failed assertion) after PT_KILL ptrace error

It might not be a debugger bug that caused the PT_KILL ptrace operation
to fail.  So emit a warning instead, and try to continue.

This patch also tries to handle the case where ptrace return -1,
but left errno set to zero.  According to the ptrace man page,
it is possible for some ptrace operations to return -1 in non-error
situations, and to detect those situations, it explains that errno
should be set prior to calling ptrace, and then checked again after.

gdb/ChangeLog:

        * darwin-nat.c (darwin_ptrace): Add documentation.
        Set errno to zero before calling ptrace.  If ptrace returns
        -1 and errno is zero, then change then return zero.
        (darwin_kill_inferior): Issue a warning instead of triggering
        a failed assertion when the PT_KILL ptrace operations returned
        nonzero.