Make the "info dll" command available on all platform.

Make the "info dll" command available on all platform.

The "info dll", an alias of the "info sharedlibrary" command, is
currently only defined in windows native versions. This patch makes
it universally available by moving the alias declaration to solib.c,
and adjusts the documentation accordingly.

Making it universally available has two benefits:
  - Windows users moving to a Unix platforms are still able to use
    the same command for getting the list of shared libraries;
  - Unix to Windows cross debuggers now provide that command also.

gdb/ChangeLog:

        * solib.c (_initialize_solib): Add "info dll" alias creation.
        * windows-nat.c (set_windows_aliases): Delete.
        (_initialize_windows_nat): Remove deprecated_init_ui_hook
        assignment.
        * NEWS: Add news entry about "info dll" now being available
        on all platforms.

gdb/doc/ChangeLog:

        * gdb.texinfo (Files): Add "info dll" documentation.
        (Cygwin Native): Remove "info dll" documentation.