fix history related APIs

Term::EditLine's history related APIs does not works.
I need a history related APIs for implementing Term::ReadLine::EditLine.
fix libcurses dependency on OpenBSD

$ make test
PERL_DL_NONLAZY=1 /usr/bin/perl "-MExtUtils::Command::MM" "-e" "test_harness(0, 'blib/lib', 'blib/arch')" t/*.t
t/1.t .. /usr/bin/perl:/usr/lib/libedit.so.4.1: undefined symbol 'tgetnum'
/usr/bin/perl:/usr/lib/libedit.so.4.1: undefined symbol 'tgoto'
/usr/bin/perl:/usr/lib/libedit.so.4.1: undefined symbol 'tgetflag'
/usr/bin/perl:/usr/lib/libedit.so.4.1: undefined symbol 'tputs'
/usr/bin/perl:/usr/lib/libedit.so.4.1: undefined symbol 'tgetent'
/usr/bin/perl:/usr/lib/libedit.so.4.1: undefined symbol 'tgetstr'
t/1.t .. ok
All tests successful.
Files=1, Tests=2,  0 wallclock secs ( 0.02 usr  0.02 sys +  0.01 cusr  0.02 csys =  0.07 CPU)
Result: PASS
$

The attached patch adds the libcurses dependency on OpenBSD (checking for openbsd in $^O). prevents the undefined symbol warnings and lets Term::ReadLine::EditLine install.
correct POD errors
remove META from dist (generated by MakeMaker)
declare minimum perl version

since this module uses our, at least 5.6 is required.  May need to
check if this actually still works on 5.6.  Also removing checks
for perl 5.005, since this dist wouldn't even work there anyway.
add gitignore
add license
add new test to MANIFEST
const-c.inc + const-xs.inc on make realclean
add license
add dist files to gitignore
update ppport.h