msvc-inval: Make handler multithread-safe.

msvc-inval: Make handler multithread-safe.

* lib/msvc-inval.h (struct gl_msvc_inval_per_thread): New type.
(gl_msvc_inval_restart, gl_msvc_inval_restart_valid): Remove
declarations.
(gl_msvc_inval_current): New declaration.
(TRY_MSVC_INVAL, CATCH_MSVC_INVAL, DONE_MSVC_INVAL) [!_MSC_VER]:
Operate on the structure returned by gl_msvc_inval_current().
* lib/msvc-inval.c (gl_msvc_inval_restart, gl_msvc_inval_restart_valid):
Remove varaiables.
(tls_index, tls_initialized): New variables.
(not_per_thread): New variable.
(gl_msvc_inval_current): New function.
(gl_msvc_invalid_parameter_handler) [!_MSC_VER]: Use the structure
returned by gl_msvc_inval_current().