curses: detect optional functions with configure probes instead of assuming ncurses #152502

Several functions in Modules/_cursesmodule.c are called unconditionally or gated only by the ncurses-specific NCURSES_EXT_FUNCS macro. Both break building or using the module against curses implementations other than ncursesw — narrow (non-wide) ncurses, and third-party curses such as NetBSD curses — even when the library provides the function.

Unconditionally-called functions that aren't universally present (no HAVE_CURSES_* guard, so a library lacking them fails to link):

• scr_dump/scr_restore/scr_init/scr_set — the whole family is absent from NetBSD curses.
• key_defined — absent from NetBSD curses (which has define_key/keyok).
• term_attrs — the X/Open wide attribute query, present only in libncursesw, so missing from a narrow -lncurses build.

Functions gated by NCURSES_EXT_FUNCS that NetBSD implements without defining that macro, so the module silently drops them:

• define_key, keyok, set_escdelay, set_tabsize, get_escdelay, get_tabsize.

Proposed change: detect each with a configure capability probe and gate on HAVE_CURSES_*.

• PY_CHECK_CURSES_FUNC probes for scr_dump (one probe gates the inseparable family), key_defined, term_attrs, define_key, keyok, set_escdelay, set_tabsize.
• get_escdelay/get_tabsize read the ESCDELAY/TABSIZE variables, which some libraries declare as extern int rather than macros, so a function probe doesn't fit. Add a small PY_CHECK_CURSES_VAR macro (tests that int x = ESCDELAY; compiles) and gate the getters on HAVE_CURSES_ESCDELAY/HAVE_CURSES_TABSIZE.

None of these are in X/Open Curses; they are ncurses extensions (NetBSD provides compatible ones), so capability-gating is the correct treatment, and a probe is more portable than conditioning on NCURSES_VERSION.

No behaviour change on standard ncursesw (all macros defined). test_curses passes and is refleak-clean on ncursesw, narrow ncurses, and NetBSD curses, under UTF-8 and 8-bit locales. test_env_queries calls curses.term_attrs() unconditionally and needs a hasattr guard for the narrow case.

Linked PRs

• gh-152502: Detect optional curses functions with configure probes #152504
• [3.15] gh-152502: Detect optional curses functions with configure probes (GH-152504) #152589
• [3.13] gh-152502: Detect optional curses functions with configure probes (GH-152504) (GH-152589) #152602
• [3.14] gh-152502: Detect optional curses functions with configure probes (GH-152504) (GH-152589) #152603
• gh-152502: Detect the curses mouse interface and is_* methods portably #152705
• [3.15] gh-152502: Detect the curses mouse interface portably (GH-152705) #152707
• [3.14] gh-152502: Detect the curses mouse interface portably (GH-152705) #152734
• [3.13] gh-152502: Detect the curses mouse interface portably (GH-152705) #152735