Refactor code to check for terminal sharing

Refactor code to check for terminal sharing

This refactors the code to check for terminal sharing.
is_gdb_terminal is exported, and sharing_input_terminal_1 is renamed,
slightly refactored, and moved to posix-hdep.c.  A new
Windows-specific implementation of this function is added to
mingw-hdep.c.

MSDN has a warning about GetConsoleProcessList

    This API is not recommended and does not have a virtual terminal
    equivalent. [...] Applications remoting via cross-platform
    utilities and transports like SSH may not work as expected if
    using this API.

However, we believe this isn't likely to be an issue for gdb.