[PATCH] fix bugs found by compiler (clang)

[Jun T <tak...@kb...>]

I get several warnings while building on macOS by using clang, and I've found
some of them are not just warnings but bugs:

datafile.c:2145:44: warning: using floating point absolute value function 'fabs' when argument is of integer type [-Wabsolute-value]
                    if (undefined || (a.type == CMPLX && fabs(imag(&a) > zero))) {
                                                         ^
# should be fabs(imag(&a)) > zero

plot2d.c:652:44: warning: implicit conversion from enumeration type 'enum DATA_TYPES' to different enumeration type 'enum coord_type' [-Wenum-conversion]
        current_plot->points[i].type = (j == 0) ? NOTDEFINED : INRANGE;
                                     ~            ^~~~~~~~~~
# 'enum coord_type' has UNDEFINED, not NOTDEFINED

../term/caca.trm:1696:12: warning: use of logical '&&' with constant operand [-Wconstant-logical-operand]
        if ((head && BOTH_HEADS) != 0) {
                  ^  ~~~~~~~~~~
# should be a single &

I believe these three are bugs.
The following two are not bugs but better to be "fixed":

mouse.c:514:13: warning: format string is not a string literal (potentially insecure) [-Wformat-security]
        sprintf(s, readout.v.string_val);
                   ^~~~~~~~~~~~~~~~~~~~

../term/pstricks.trm:1079:49: warning: the value of the size argument in 'strncat' is too large, might lead to a buffer overflow [-Wstrncat-size]
            strncat(patfill, ",fillcolor=PST@BGCOLOR", sizeof(patfill));
                                                       ^~~~~~~~~~~~~~~

The next one is not a problem but the waring can be easily silenced:

plot3d.c:1041:40: warning: equality comparison with extraneous parentheses [-Wparentheses-equality]
            } else if ((this_plot->plot_style == CIRCLES)) {
                        ~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~

Jun