Use G_DIR_SEPARATOR_S rather than "/" as a pathname separator in format

Use G_DIR_SEPARATOR_S rather than "/" as a pathname separator in format
strings used to generate pathnames.

Move the definition of PF_DIR from <epan/epan.h> to <epan/filesystem.h>,
so that files requiring only the definition of PF_DIR don't have to
include <epan/epan.h>, and get rid of no-longer-necessary includes of
<epan/epan.h>.

Add a routine to get the directory for "system files" such as
"/etc/ethers" - it's "/etc" on UNIX, and the datafile directory on
Windows (as there's no "/etc" on Windows).  Use that to construct the
pathname of the ethers and ipxnet files.

git-svn-id: http://anonsvn.wireshark.org/wireshark/trunk@4056 f5534014-38df-0310-8fa8-9805f1628bb7