Add missing va_end() calls.

Fixes Coverity CIDs 59620 and 59621.
Change 'char *' to 'const char *' in headers included by plugins.

The change propagate to several classes using the values.
This get rid of all warnings in the colors plugin, and several
warnings in other plugins.

This reduce the amount of warnings and bring the code closer to HV 4.5.
Enable format related compiler warnings and errors.

Compile by default with -Wformat -Werror=format-security to
ensure that bugs that would break hardening build on Debian are not
introduced in the code.
Enable setting the required kernel.shmmax at first run (under debian)

This patch allows the setting the required kernel.shmmax at first run (under debian).
Without it, cinelerra will ask for the correct kernel.shmmax after
the first installation which is fairly confusing for the user.