runtime/stp_string.c: Update _stp_text_str to handle '\0' and take a buffer flag
runtime/stp_string.h: Update _stp_text_str prototype
tapset/linux/aux_syscalls.stp: Update calls to _stp_text_str
tapset/linux/conversions.stp: Add user_buffer_quoted and _warn tapset functions
tapset/linux/syscalls2.stp: Update syscall.write to use user_buffer_quoted
tapset/string.stp: Update calls to _stp_text_str
tapset/uconversions.stp: Add kernel_buffer_quoted and _warn tapset functions