Modified interfaces to getting smb password entries from

Modified interfaces to getting smb password entries from
get_smbpwd_entry (now an internal function to smbpass.c)
to a more UNIX-like :

getsmbpwnam() - get entry by name.
getsmbpwuid() - get entry by uid.

Changed the type returned by the smbpasswd enumeration
functions to be a void * so that people don't come to
depend on it being a FILE *.

These abstractions should make it much easier to
replace the smbpasswd file with a better backend
in future.

Other files changed are to match the above changes.

Jeremy.
(This used to be commit 1161cfb7f2b0d5a6d3e2b524a14a6f325ce70efb)