Add PS_NAME attribute in info mapping if possible. Needed to
facilitate compatibility with code relying on behaviour of older
versions of FreeType.

Background: modern FreeType implementations return the FONT_FAMILY and
FONT_SUBFAMILY SFNT attributes as family and style in the mapping
returned by the info() function; older versions (at least FreeType
2.1.9 and older) returned PS_NAME and a string derived from other data
for those fields.

The problem was noticed when a Roxen customer wanted to update their
Roxen CMS installation from 4.5 (based on on Pike7.4 with
FreeType2.1.9) to a more modern version. Additional fixes in Roxen
will be required for full compatibility, but this Pike change makes
the necessary data available to enable that. (Roxen internal note:
RT#21337.)
release number bumped to 796 by export.pike
release number bumped to 797 by export.pike
initialize pike_frame->num_args even in special cases
Get rid of some harmless valgrind warnings
activate __CHECKER__ and CLEANUP when --with-valgrind
TURBO2-80: Do not crash when decoding certain PNG files.

The aggregate and n++ was not done if the PNG was (slightly) truncated
Build: sprshd doesn't seem to like I/O redirection.
Backend: Fixed typo in out of band data handling.

Thanks to Chris Angelico <rosuav@gmail.com> for the report.

Fixes [LysLysKOM 20481103]/[Pike mailing list 13683].
Backend: Improved thread safety in find_call_out() et al.

backend_find_call_out() called is_eq() (which may call Pike code
and release the interpreter lock) in a PROTECT_CALL_OUTS() context.
This could cause call_out operations performed in other threads to
either (no debug) mess with the hash table being traversed or (with
debug) cause the fatal "Recursive call in call_out module.".