[gdbsupport] Add gdb::{waitpid,read,write,close}

[gdbsupport] Add gdb::{waitpid,read,write,close}

We have gdb::handle_eintr, which allows us to rewrite:
...
  ssize_t ret;
    do
      {
        errno = 0;
        ret = ::write (pipe[1], "+", 1);
      }
    while (ret == -1 && errno == EINTR);
...
into:
...
  ssize_t ret = gdb::handle_eintr (-1, ::write, pipe[1], "+", 1);
...

However, the call to write got a bit mangled, requiring effort to decode it
back to its original form.

Instead, add a new function gdb::write that allows us to write:
...
  ssize_t ret = gdb::write (pipe[1], "+", 1);
...

Likewise for waitpid, read and close.

Tested on x86_64-linux.