lib/param: add a fixed unified lpcfg_string_{free,set,set_upper}() infrastructure

lib/param: add a fixed unified lpcfg_string_{free,set,set_upper}() infrastructure

This reduces the memory footprint of empty string options.

smbd -d1 -i with 1400 shares in smb.conf under x64 valgrind massif before this
patch has 7,703,392 bytes peak memory consumption and after this patch
3,321,200 bytes.

This fixes a regression introduced by commit
2dd7c890792cf12049ec13b88aa4e9de23035f9d.

BUG:

Bug: https://bugzilla.samba.org/show_bug.cgi?id=11625
Signed-off-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Volker Lendecke <vl@samba.org>
Autobuild-User(master): Volker Lendecke <vl@samba.org>
Autobuild-Date(master): Mon Nov 30 17:41:28 CET 2015 on sn-devel-104