Original patch by Raymond Cole with some modifications, thanks!

- drw: minor improvement to the nomatches cache
- overhaul utf8decoding and render invalid utf8 sequences as U+FFFD.

Thanks NRK for these improvements!

Caught by -pedantic implying -Wstrict-prototypes for OpenBSD's 16.0.6 Clang.

The Makefile used to suppress output (by using @), so this target made sense at
the time.

But the Makefile should be simple and make debugging with less abstractions or
fancy printing.  The Makefile was made verbose and doesn't hide the build
output, so remove this target.

Prompted by a question on the mailing list about the options target.

From sigaction(2):
A child created via fork(2) inherits a copy of its parent's signal dispositions.
During an execve(2), the dispositions of handled signals are reset to the default;
the dispositions of ignored signals are left unchanged.

This refused to start directly some programs from configuring in config.h:

static Key keys[] = {
	MODKEY,                       XK_o,      spawn,          {.v = cmd } },
};

Some reported programs that didn't start were: mpv, anki, dmenu_extended.

Reported by pfx.
Initial patch suggestion by Storkman.

SA_NOCLDWAIT is marked as XSI in the posix spec [0] and FreeBSD and NetBSD
seems to more be strict about the feature test macro [1].

so update the macro to use _XOPEN_SOURCE=700L instead, which is equivalent to
_POSIX_C_SOURCE=200809L except that it also unlocks the X/Open System
Interfaces.

[0]: https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/signal.h.html#tag_13_42
[1]: https://lists.suckless.org/dev/2302/35111.html

Tested on:
* NetBSD 9.3 (fixed).
* FreeBSD 13 (fixed).
* Void Linux musl.
* Void Linux glibc.
* OpenBSD 7.2 (stable).
* Slackware 11.

Reported-by: beastie <pufferfish@riseup.net>