curses A_ITALIC build fix

curses A_ITALIC build fix

My /usr/include/curses.h has various A_attribute macros but A_ITALIC
isn't one of them.  Compiling cursmisc.c failed because one of the
uses of that wasn't guarded by #ifdef A_ITALIC.  Instead of adding the
ommitted #if, substitute A_UNDERLINE for A_ITALIC when that's missing.

The select attribute menu when adding a menu color or a status hilite
now shows an entry for italic that's underlined (as expected) but the
underline entry itself does not display any sort of attribute.  I
didn't pursue that.