Remove certain AFM tags

Here we remove lines from the AFM file based on a set of regular expressions.  At present we remove only "^Character" as this is the AFM tag that causes ugly warnings when used by R.  Others may be added in the future as needed.

Note that gzcopy_exclude() uses text mode to read and write the files so it can do the matching--the old gzcopy() used binary.  I have not run into a problem with this on my linux workstation but we may want to test under windows to make sure there are no gotchas.  Also in gzcopy_exclude() we use useBytes=TRUE in the grepl() call in order to avoid the warnings this generates any time a non-ascii character appears.  We might think about changing all grepl() calls to work this way at some point.
I just realized that `n` is the number of lines to read, not bytes as it is with `readBin`. Can you remove `n` here and remove the comment above about file size?

Ideally, it would be good to set the file size limit to 100MB, but I don't think that's easily done with `readLines`.