maint: copy FDL from gnulib instead of using it as module

Since the previous gnulib update, bootstrap outputs this warning:

  Notice from module fdl:
    Don't use this module! Instead, copy the referenced license file \
    into your version control repository.

See gnulib commit:
  https://git.sv.gnu.org/cgit/gnulib.git/commit/?id=88fc5afbccc9

* bootstrap.conf (gnulib_modules): Remove 'fdl'.
* doc/fdl.texi: Add file as a copy of 'gnulib/doc/fdl.texi'.
* doc/.gitignore (/fdl.texi): Remove entry.
* cfg.mk (FILTER_LONG_LINES): Add pattern for the 'fdl.texi' file.

bootstrap.conf

@@ -82,7 +82,6 @@ gnulib_modules="
   fcntl-safer
   fd-reopen
   fdatasync
-  fdl
   fdopen
   fdutimensat
   file-has-acl

cfg.mk

@@ -327,11 +327,13 @@ sc_prohibit-gl-attributes:
 
 # Look for lines longer than 80 characters, except omit:
 # - urls
+# - the fdl.texi file copied from gnulib,
 # - the help2man script copied from upstream,
 # - tests involving long checksum lines, and
 # - the 'pr' test cases.
 FILTER_LONG_LINES =						\
   \|^[^:]*NEWS:.*https\{,1\}://| d;					\
+  \|^[^:]*doc/fdl.texi:| d;					\
   \|^[^:]*man/help2man:| d;					\
   \|^[^:]*tests/misc/sha[0-9]*sum.*\.pl[-:]| d;			\
   \|^[^:]*tests/pr/|{ \|^[^:]*tests/pr/pr-tests:| !d; };

doc/.gitignore

@@ -17,7 +17,6 @@
 /coreutils.toc
 /coreutils.tp
 /coreutils.vr
-/fdl.texi
 /gendocs_template
 /gendocs_template_min
 /parse-datetime.texi