nss_files: Add generic code for set*ent, end*ent and file open

nss_files: Add generic code for set*ent, end*ent and file open

This reduces RSS usage if nss_files is not actually used, and can
be used later to make NSS data thread-specific.  It also results in
a small code size reduction.

Before:

   text	   data	    bss	    dec	    hex	filename
   2288	      0	     72	   2360	    938	nss/files-alias.os
   1807	      0	     72	   1879	    757	nss/files-ethers.os
   1371	      0	     72	   1443	    5a3	nss/files-grp.os
   6246	      0	     72	   6318	   18ae	nss/files-hosts.os
    869	      0	      0	    869	    365	nss/files-initgroups.os
    666	      0	      0	    666	    29a	nss/files-init.os
   1934	      0	      0	   1934	    78e	nss/files-netgrp.os
   2353	      0	     72	   2425	    979	nss/files-network.os
   2130	      0	     72	   2202	    89a	nss/files-proto.os
   1372	      0	     72	   1444	    5a4	nss/files-pwd.os
   2124	      0	     72	   2196	    894	nss/files-rpc.os
   2265	      0	     72	   2337	    921	nss/files-service.os
   1125	      0	     72	   1197	    4ad	nss/files-sgrp.os
   1124	      0	     72	   1196	    4ac	nss/files-spwd.os

After:

   text	   data	    bss	    dec	    hex	filename
   2040	      0	      0	   2040	    7f8	nss/files-alias.os
   1599	      0	      0	   1599	    63f	nss/files-ethers.os
   1155	      0	      0	   1155	    483	nss/files-grp.os
   6010	      0	      0	   6010	   177a	nss/files-hosts.os
    869	      0	      0	    869	    365	nss/files-initgroups.os
    666	      0	      0	    666	    29a	nss/files-init.os
   1934	      0	      0	   1934	    78e	nss/files-netgrp.os
   2129	      0	      0	   2129	    851	nss/files-network.os
   1914	      0	      0	   1914	    77a	nss/files-proto.os
   1156	      0	      0	   1156	    484	nss/files-pwd.os
   1908	      0	      0	   1908	    774	nss/files-rpc.os
   2057	      0	      0	   2057	    809	nss/files-service.os
    909	      0	      0	    909	    38d	nss/files-sgrp.os
    908	      0	      0	    908	    38c	nss/files-spwd.os
   1090	      0	      8	   1098	    44a	nss/nss_files_data.os

27674 code bytes before, 26344 code bytes after, so it is an overall
win despite the extra initialization code.

Reviewed-by: Adhemerval Zanella  <adhemerval.zanella@linaro.org>