more consistent file naming

[Bruno Haible]

For consistency and memoizability, I renamed three files:

2007-04-27  Bruno Haible  <[EMAIL PROTECTED]>

        * lib/sysexits_.h: Renamed from lib/sysexit_.h.
        * modules/sysexits (Files, Makefile.am): Update.

        * lib/sys_socket_.h: Renamed from lib/socket_.h.
        * modules/sys_socket (Files, Makefile.am): Update.

        * lib/sys_stat_.h: Renamed from lib/stat_.h.
        * modules/sys_stat (Files, Makefile.am): Update.