remove support for the 'L' (fmt/flow-paragraph) command

* sed/Makefile.am (sed_SOURCES): Remove fmt.c.
* sed/execute.c (execute_program):
* sed/fmt.c: Remove file.
* sed/sed.h (fmt): Remove declaration.
* doc/sed.texi: Remove documentation for 'L' command.
* doc/sed-in.texi: Likewise.
* NEWS (Feature removal): Document it.
Prompted by the report from Jodie Cunningham that using this
command with a large number could cause sed to segfault:
https://bugs.launchpad.net/ubuntu/+source/sed/+bug/1400575
build: build-aux/texi2dvi: remove file

* build-aux/texi2dvi: Remove file. It was out of date and could
no longer parse our .texi files.
* Makefile.am (EXTRA_DIST): Don't list it here.
* cfg.mk: Remove an exemption for that file.
* doc/Makefile.am (TEXI2DVI): Remove definition.
maint: fix "make distcheck" failure

* Makefile.am (EXTRA_DIST): List THANKS.in, not THANKS.
Add .mailmap, too.
build: update build tool version requirements

* bootstrap.conf (buildreq): Blindly update tool-version number
requirements from those required by coreutils.
maint: remove po/sed.pot from version control

* po/sed.pot: Remove file.  It is unnecessary and undesirable to
version-control a file like this (mechanically-derived).
maint: update .gitignore

* .gitignore: Ignore build artifacts.
maint: begin to use "size_t" more, where appropriate

* sed/execute.c (do_list): Update some "int" locals to "size_t",
and combine a few declarations with needlessly separate initializations.
maint: enable sc_prohibit_strncpy check

* cfg.mk (exclude_file_name_regexp--sc_prohibit_strncpy): Remove
its exemption.
* sed/compile.c (compile_program): Remove two misuses of strncpy;
use memcpy instead.
* sed/execute.c (do_list): Likewise, for one instance.