USES=desktop-file-utils, INSTALLS_ICONS, and pkg

[Don Lewis]

I've been working on a port update and happened to notice that when I
installed an updated package the entries under the Applications menu on
my desktop were not getting updated.

Looking at /usr/ports/Mk/Uses/desktop-file-utils.mk, I see that it adds
"@desktop-file-utils" to PLIST_FILES, and I do see this in .PLIST.mktmp
when I build the port.  I don't see it in the +MANIFEST file in the
package .txz file.  Digging through the source for pkg, I don't see
anything in the code for handling @desktop-file-utils.

Something else that I don't see in the source for pkg is anything that
handles @rmtry, which INSTALL_ICONS wants to add to the plist.

Neither of these @things is documented in pkg-create(8).  Reading this
man page, I see that @exec and @unexec are deprecated, and these are
used fairly heavily in the ports framework.