Revisit of aligment hints for setting the offset of header and data section

The setting is based on the followings.

Each time ncmpi__enddef() is called, use env variable and MPI info
object to recalculate ncp->v_align and ncp->r_align, which are later
used to calculate the file starting offsets for fix-sized variable
section and record variable section.

    if (env variable of v_align is set)
        ncp->v_align = env value
    else if (MPI info hint of v_align is set)
        ncp->v_align = hint value
    else if (v_align passed to ncmpi__enddef() > 0)
        ncp->v_align = v_align + 4 - v_align % 4;
    else /* Not set by user, <= 0 */
        ncp->v_align = 0;

if has fix-sized variables
    if ncp->v_align > 0    /* v_align is set by users */
        use it
    else
        ncp->v_align = default 512

    * use ncp->v_align to set ncp->begin_var

    if ncp->r_align > 0    /* r_align is set by users */
        use it
    else
        ncp->r_align = default 4

    * use ncp->r_align to set ncp->begin_rec

else
    discard ncp->v_align
    discard ncp->v_minfree

    if ncp->r_align > 0     /* r_align is set by users */
        use it
    else
        ncp->r_align = default 512

    * use ncp->r_align to set ncp->begin_rec
    * set ncp->begin_var == ncp->begin_rec

Do not reuse ncp->v_align or ncp->r_align in next ncmpi_enddef() !

Author: wkliao