Some rewriting inspired by suggestions of overflow bugs and other problems

Some rewriting inspired by suggestions of overflow bugs and other problems
with this code.

denyusers.c and allowusers.c have a fair amount of nearly identical code.
I moved these functions to a new file, called usersfile.[ch].  I didn't
really want to add another source file, but....

The list of allowed/denied users were formerly stored as whitespace
separated tokens in a single char buffer.  The code used strstr() to
search for matches.  This was awkward because the search key had to
be surrounded by space characters.  The new code uses and array
of char buffers -- one for each name.  The new code uses qsort() and
bsearch().

The old code was more robust with the input file.  You could put multiple
names on a single line.  Due to the way the names were stored, it didn't
matter.  The new code is less robust.  It requires one name per line,
and no leading whitespace.

Names longer than NAMELEN are now truncated.

Added an openlog() call and Removed LOG_USER from syslog() calls.

No longer use scanf() in some places.

There is some weird logic about what to do if an allow/deny file
does not exist, or exists but cannot be read, or was read once but
cannot be read later.  I hope it still works the same.

replaced some strcpy() calls with memset() and strncpy().

Fixed a strange usage of ServerArray[] that started at index 1?

#ifdefd-out some debugging code that included fprintfs to stderr, but
was apparently never called.

Rewrote some signal handling stuff to use sigaction() (and fall back
to signal()) and removed calls to sigvec().