Rename buffer_ routines to ws_buffer_ to avoid name collisions.

In particular, epan/wslua/lrexlib.c has its own buffer_ routines,
causing some linker warnings on some platforms, as reported in bug
10332.

(Not to be backported to 1.12, as that would change the API and ABI of
libwsutil and libwiretap.  We should also make the buffer_ routines in
epan/wslua/lrexlib.c static, which should also address this problem, but
the name change avoids other potential namespace collisions.)

Change-Id: I1d42c7d1778c7e4c019deb2608d476c52001ce28
Reviewed-on: https://code.wireshark.org/review/3351
Reviewed-by: Guy Harris <guy@alum.mit.edu>