posix: simplify thread error handler
vlc_strerror() and vlc_strerror_c() convenience wrappers

Those are more flexible than the GNU-specific %m format specifier to
print standard error messages. They are also less likely to format a
clobbered error number.
win32: implement vlc_strerror and vlc_strerror_c
src: use vlc_strerror_c() for log messages
file: use vlc_strerror()

This fixes "%m" in error dialogs on non-GNU platforms.
libav: use vlc_strerror_c()
DTV: use vlc_strerror_c()
DV: use vlc_strerror_c()
DVB (old): use vlc_strerror_c()
MMS: use vlc_strerror_c()
MTP: use vlc_strerror() & vlc_strerror_c()
linsys: use vlc_strerror_c()
OSS: use vlc_strerror_c()
RTP: use vlc_strerror_c()
XCB: use vlc_strerror_c()
VDR: use vlc_strerror() and vlc_strerror_c()
samba: use vlc_strerror_c()
shm: use vlc_strerror_c()
V4L: use vlc_strerror_c()
Linux FB: use vlc_strerror_c()