OpenSystemsDevelopment / qpdf

Handle optional choices in addition to required choices. Refactor the
way help options are added to completion to make it work with optional
help choices.

* Make it a runtime error, not a logic error
* Include additional information
* Capture it properly in checkLinearization

Create a computationally and memory efficient implementation of name
and number trees that does binary searches as intended by the data
structure rather than loading into a map, which can use a great deal
of memory and can be very slow.

Turns out unreadCh is much more efficient than seek(-1, SEEK_CUR).
Update comments and code to reflect this.

We need to run qpdf --show-crypto.

This is all that ever worked. The test suite was trying to do
something different from ClosedFileInputSource.

When possible, use `for (auto&` or `for (auto const&` when iterating
using C++-11 style iterators.

Build fails on gcc 4.8 since version 9.1.1 and commit
752416554086d5d34323bc14164d5084db83cfbd:

libtests/cxx11.cc: In function 'void do_regex()':
libtests/cxx11.cc:347:42: error: 'strlen' is not a member of 'std'
     std::cregex_iterator m3(str7, str7 + std::strlen(str7), expr4);
                                          ^

To fix the build failure, add missing include on cstring

Fixes:
 - http://autobuild.buildroot.org/results/ad7fb68ae87850a85509eed80fd0cae8721b10c5

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>

This commit adds the preserve_eol flags but doesn't implement EOL
preservation yet.

If QPDF_CRYPTO_PROVIDER is set, just run the tests for the given
provider. This is to support cases of running the entire test suite
for each provider. If QPDF_CRYPTO_PROVIDER is not set, run the tests
that exercise the cyrpto provider for each available provider.