Fix linknamespace errors and local-plt-usages in nss_files.

After commit f9c8b11e
"nss: Access nss_files through direct references",
when building with -Os, multiple conform/.../linknamespace tests
and elf/check-localplt are failing:
Extra PLT reference: libc.so: fgetc_unlocked
Extra PLT reference: libc.so: getline

Or e.g.:
[initial] glob -> [libc.a(glob.o)] __getpwnam_r -> [libc.a(getpwnam_r.o)] __nss_database_custom -> [libc.a(nsswitch.o)] __nss_module_get_function -> [libc.a(nss_module.o)] __nss_files_functions -> [libc.a(nss_files_functions.o)] _nss_files_endaliasent -> [libc.a(files-alias.o)] feof_unlocked
[initial] glob -> [libc.a(glob.o)] __getpwnam_r -> [libc.a(getpwnam_r.o)] __nss_database_custom -> [libc.a(nsswitch.o)] __nss_module_get_function -> [libc.a(nss_module.o)] __nss_files_functions -> [libc.a(nss_files_functions.o)] _nss_files_endaliasent -> [libc.a(files-alias.o)] fgetc_unlocked
[initial] glob -> [libc.a(glob.o)] __getpwnam_r -> [libc.a(getpwnam_r.o)] __nss_database_custom -> [libc.a(nsswitch.o)] __nss_module_get_function -> [libc.a(nss_module.o)] __nss_files_functions -> [libc.a(nss_files_functions.o)] _nss_files_endnetgrent -> [libc.a(files-netgrp.o)] getline

This patch is using the hidden symbols where possible.
Instead of fputc_unlocked, __putc_unlocked is used.
(Compare to commit eeaa19f7
"mntent: Use __putc_unlocked instead of fputc_unlocked")