Make the maximum number of files a configure-time setting, for operating

Make the maximum number of files a configure-time setting, for operating
systems (OSX) that report 2^31-1 for the upper limit on file descriptors.

The default is 4096, which will support 1365 simultaneous clients.
The new --with-maxfiles=N option can override the default value of 4096.

Also, make the FD_SETSIZE stuff applicable only to Solaris, which needs
it to use the "big" select function.

git-svn-id: svn+ssh://src.apple.com/svn/cups/cups.org/trunk@3349 7a7537e8-13f0-0310-91df-b6672ffda945