grep: handle lines longer than INT_MAX on more systems

When trying to exercize some long-line-handling code, I ran these
commands:
  $ dd bs=1 seek=2G of=big < /dev/null; grep -l x big; echo $?
  grep: big: Invalid argument
  2
grep should not have issued that diagnostic, and it should
have exited with status 1, not 2.  What happened?
grep read the 2GiB of NULs, doubled its buffer size,
copied the 2GiB into the new 4GiB buffer, and proceeded
to call "read" with a byte-count argument of 2^32.
On at least Darwin 12.5.0, that makes read fail with EINVAL.
The solution is to use gnulib's safe_read wrapper.
* src/main.c: Include "safe-read.h"
(fillbuf): Use safe_read, rather than bare read.  The latter
cannot handle a read size of 2^32 on some systems.
* bootstrap.conf (gnulib_modules): Add safe-read.
* tests/long-line-vs-2GiB-read: New file.
* tests/Makefile.am (TESTS): Add it.
* NEWS (Bug fixes): Mention it.
maint: indent cpp directives to reflect nesting

* src/pcresearch.c: Insert spaces after a few "#", to indent
cpp directives to reflect their nesting.
pcre: avoid a nominal leak

* src/pcresearch.c (Pcompile)[HAVE_LIBPCRE && !PCRE_STUDY_JIT_COMPILE]:
We would leak "re" if built with HAVE_LIBPCRE but without
PCRE_STUDY_JIT_COMPILE.  Move the free out one level.
maint: add comments and split some long lines

* src/main.c (do_execute): Add a comment.
Split some lines longer than 80 bytes.