net-fs/nfs-utils: Handle quoted export directories

net-fs/nfs-utils: Handle quoted export directories

exports(5) says

       Blank lines are ignored.  A pound sign ("#") introduces a comment
       to the end of the line. Entries may be continued across newlines
       using a backslash. If an export name contains spaces it  should
       be quoted using double quotes. You can also specify spaces or
       other unusual character in the export name using a backslash
       followed by the character code as three octal digits.

Thus, allow an optional single double-quote character before the /.

Closes: https://bugs.gentoo.org/675648
Signed-off-by: Matt Turner <mattst88@gentoo.org>