[gdb] Handle EINTR in run_under_shell

When building gdb with -O2 -fsanitize=thread and running test-case
gdb.base/bg-exec-sigint-bp-cond.exp, I run into:
...
(gdb) c&^M
Continuing.^M
(gdb) Quit^M
(gdb) quit_count=1
^M
Breakpoint 2, foo () at bg-exec-sigint-bp-cond.c:23^M
23        return 0;^M
FAIL: $exp: no force memory write: \
  SIGINT does not interrupt background execution
...

What happens is that:
- the breakpoint hits
- while evaluating the condition of the breakpoint,
  $_shell("kill -INT <pid-of-gdb>") is called, handled by run_under_shell
- in run_under_shell, a vfork is issued
- in the vfork child, execl executes the kill command
- in the vfork parent, waitpid is called to wait for the result of the kill
  command
- waitpid returns -1 with errno set to EINTR
- run_under_shell doesn't check the result of waitpid, and returns the
  value of local variable status.  Since waitpid returned -1, status was
  not assigned a value, so it's uninitialized, and happens to be
  non-zero
- the breakpoint condition evaluates to true, because
  $_shell("kill -INT <pid-of-gdb>") != 0
- the breakpoint triggers a stop, which the test-case doesn't expect.

Fix this by using gdb::handle_eintr to call waitpid in run_under_shell.

Also handle the case that waitpid returns an error other than EINTR, using
perror_with_name.

Tested on x86_64-linux.

Approved-By: Tom Tromey <tom@tromey.com>

PR gdb/30695
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=30695

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