Check return values of functions declared with warn_unused_result

Check return values of functions declared with warn_unused_result
attribute in GLIBC 2.8.
* cli/cli-cmds.c (pwd_command): Check return value from getcwd.
* inflow.c (check_syscall): New function.
(new_tty): Use check_syscall to check return values from open and dup.
* linux-nat.c (linux_nat_info_proc_cmd): Check return value from fgets.
* main.c (captured_main): Call cwd after setting up gdb_stderr;
check for errors from getcwd.
* mi/mi-cmd-env.c (mi_cmd_env_pwd): Check return value from getcwd.
* ui-file.c (stdio_file_write): Ignore return value from fwrite.
(stdio_file_fputs): Same.
* utils.c (internal_vproblem): abort if last-ditch error message
write fails.