Enhance builtin bytecode_data to support embedded NULs in character strings.

Enhance builtin bytecode_data to support embedded NULs in character strings.
While NASM doesn't allow this, GAS does.

While we're here, greatly clean up GAS data bytecode creation by no longer
building intermediate valparam list.

* bytecode.h (yasm_dv_create_string): Add length parameter.
(yasm_bc_create_data): Add append_zero parameter for new ability to append
a single ero byte after each data value.  This is used by the GAS .asciz
directive.
* bytecode.c (bytecode_data, ...): Implement the above.

* gas-bison.y (gas_define_strings, gas_define_data)
(gas_define_leb128): Remove; replace in usage with direct calls to bytecode
functions.  Add str, dataval, and datavalhead to parser union.  Add new
dirvals, which has valparams type, and change strvals and datavals to
datavals type.
* gas-token.re: Use new str type where STRING token is generated.

* nasm-bison.y: Add str type to union, and use for STRING token.
* nasm-token.re: Use new str type where STRING token is generated.

* coff-objfmt.c (win32_objfmt_directive): Adjust for updates to
bytecode_data.

* strzero.asm: Simple test for NUL in GAS string.

svn path=/trunk/yasm/; revision=1293