build: improved handling of <execinfo.h>, BSD <net/if.h>

FreeBSD and OpenBSD have a <net/if.h> that is not self-contained;
and mingw lacks the header altogether.  But gnulib has just taken
care of that for us, so we might as well simplify our code.  In
the process, I got a syntax-check failure if we don't also take
the gnulib execinfo module.

* .gnulib: Update to latest, for execinfo and net_if.
* bootstrap.conf (gnulib_modules): Add execinfo and net_if modules.
* configure.ac: Let gnulib check for headers.  Simplify check for
'struct ifreq', while also including enough prereq headers.
* src/internal.h (IF_NAMESIZE): Drop, now that gnulib guarantees it.
* src/nwfilter/nwfilter_learnipaddr.h: Use correct header for
IF_NAMESIZE.
* src/util/virnetdev.c (includes): Assume <net/if.h> exists.
* src/util/virnetdevbridge.c (includes): Likewise.
* src/util/virnetdevtap.c (includes): Likewise.
* src/util/logging.c (includes): Assume <execinfo.h> exists.
(virLogStackTraceToFd): Handle gnulib's fallback implementation.

bootstrap.conf

@@ -38,6 +38,7 @@ count-one-bits
 crypto/md5
 dirname-lgpl
 environ
+execinfo
 fclose
 fcntl
 fcntl-h
@@ -70,6 +71,7 @@ manywarnings
 mkstemp
 mkstemps
 mktempd
+net_if
 netdb
 nonblocking
 openpty