Fix texinfo syntax errors

Fix the following texinfo syntax errors:

ledger/doc/ledger3.texi:7878: @itemx must follow @item
ledger/doc/ledger3.texi:7939: @itemx must follow @item
ledger/doc/ledger3.texi:7944: @itemx must follow @item
BUILD_WEB_DOCS implies BUILD_DOCS

This fixes the build when -DBUILD_WEB_DOCS=1 is specified wihout
specifying -DBUILD_DOCS.  This fails because man2html/groff are
not found because the find_program command for these programs
is only run if BUILD_DOCS is enabled.
make --columns default to terminal width, as returned by ioctl()

If set, the COLUMNS environment variable will take precedence over terminal
width. However, please note that COLUMNS is usually *not* exported by shells to
child processes, so in most cases COLUMNS will be undefined for ledger---hence
the motivation for this change.

Terminal width is queried using ioctl() on stdin. For the sake of portability
the querying is done only on platform where ioctl() is detected as supported at
compile-time.
Typo fix

Thanks to Dominik Honnef for spotting it.
Fix numbers from example

Thanks to Dominik Honnef.
Update ledger3.texi

Fixed typos and clarified CSV import behaviour with regard to column headers (I haven't checked the code, but I noticed the programme behaves this way).
Update ledger3.texi

Improved wording
Merge pull request #327 from myqlarson/patch-1

Update ledger3.texi
Some minor changes
Merge branch 'master' of https://github.com/ledger/ledger